bgpd: Fix errors handling for MP/GR capabilities as dynamic capability

bgpd: Fix errors handling for MP/GR capabilities as dynamic capability

When receiving a MP/GR capability as dynamic capability, but malformed, do not
forget to advance the pointer to avoid hitting infinity loop.

After:
```
Mar 29 11:15:28 donatas-laptop bgpd[353550]: [GS0AQ-HKY0X] 127.0.0.1 rcv CAPABILITY
Mar 29 11:15:28 donatas-laptop bgpd[353550]: [JTVED-VGTQQ] 127.0.0.1(donatas-pc): CAPABILITY has action: 1, code: 5, length 0
Mar 29 11:15:28 donatas-laptop bgpd[353550]: [JTVED-VGTQQ] 127.0.0.1(donatas-pc): CAPABILITY has action: 1, code: 0, length 0
Mar 29 11:15:28 donatas-laptop bgpd[353550]: [HFHDS-QT71N][EC 33554494] 127.0.0.1(donatas-pc): unrecognized capability code: 0 - ignored
Mar 29 11:15:28 donatas-laptop bgpd[353550]: [JTVED-VGTQQ] 127.0.0.1(donatas-pc): CAPABILITY has action: 0, code: 0, length 0
Mar 29 11:15:28 donatas-laptop bgpd[353550]: [HFHDS-QT71N][EC 33554494] 127.0.0.1(donatas-pc): unrecognized capability code: 0 - ignored
Mar 29 11:15:28 donatas-laptop bgpd[353550]: [JTVED-VGTQQ] 127.0.0.1(donatas-pc): CAPABILITY has action: 0, code: 0, length 0
Mar 29 11:15:28 donatas-laptop bgpd[353550]: [HFHDS-QT71N][EC 33554494] 127.0.0.1(donatas-pc): unrecognized capability code: 0 - ignored
Mar 29 11:15:28 donatas-laptop bgpd[353550]: [JTVED-VGTQQ] 127.0.0.1(donatas-pc): CAPABILITY has action: 0, code: 0, length 1
Mar 29 11:15:28 donatas-laptop bgpd[353550]: [HFHDS-QT71N][EC 33554494] 127.0.0.1(donatas-pc): unrecognized capability code: 0 - ignored
Mar 29 11:15:28 donatas-laptop bgpd[353550]: [JTVED-VGTQQ] 127.0.0.1(donatas-pc): CAPABILITY has action: 1, code: 1, length 10
Mar 29 11:15:28 donatas-laptop bgpd[353550]: [Z1DRQ-N6Z5F] 127.0.0.1(donatas-pc): Dynamic Capability MultiProtocol Extensions afi/safi invalid (bad-value/unicast)
```

Before:
```
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [JTVED-VGTQQ] 127.0.0.1(donatas-pc): CAPABILITY has action: 1, code: 1, length 10
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [Z1DRQ-N6Z5F] 127.0.0.1(donatas-pc): Dynamic Capability MultiProtocol Extensions afi/safi invalid (bad-value/unicast)
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [JTVED-VGTQQ] 127.0.0.1(donatas-pc): CAPABILITY has action: 1, code: 1, length 10
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [Z1DRQ-N6Z5F] 127.0.0.1(donatas-pc): Dynamic Capability MultiProtocol Extensions afi/safi invalid (bad-value/unicast)
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [JTVED-VGTQQ] 127.0.0.1(donatas-pc): CAPABILITY has action: 1, code: 1, length 10
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [Z1DRQ-N6Z5F] 127.0.0.1(donatas-pc): Dynamic Capability MultiProtocol Extensions afi/safi invalid (bad-value/unicast)
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [JTVED-VGTQQ] 127.0.0.1(donatas-pc): CAPABILITY has action: 1, code: 1, length 10
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [Z1DRQ-N6Z5F] 127.0.0.1(donatas-pc): Dynamic Capability MultiProtocol Extensions afi/safi invalid (bad-value/unicast)
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [JTVED-VGTQQ] 127.0.0.1(donatas-pc): CAPABILITY has action: 1, code: 1, length 10
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [Z1DRQ-N6Z5F] 127.0.0.1(donatas-pc): Dynamic Capability MultiProtocol Extensions afi/safi invalid (bad-value/unicast)
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [JTVED-VGTQQ] 127.0.0.1(donatas-pc): CAPABILITY has action: 1, code: 1, length 10
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [Z1DRQ-N6Z5F] 127.0.0.1(donatas-pc): Dynamic Capability MultiProtocol Extensions afi/safi invalid (bad-value/unicast)
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [JTVED-VGTQQ] 127.0.0.1(donatas-pc): CAPABILITY has action: 1, code: 1, length 10
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [Z1DRQ-N6Z5F] 127.0.0.1(donatas-pc): Dynamic Capability MultiProtocol Extensions afi/safi invalid (bad-value/unicast)
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [JTVED-VGTQQ] 127.0.0.1(donatas-pc): CAPABILITY has action: 1, code: 1, length 10
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [Z1DRQ-N6Z5F] 127.0.0.1(donatas-pc): Dynamic Capability MultiProtocol Extensions afi/safi invalid (bad-value/unicast)
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [JTVED-VGTQQ] 127.0.0.1(donatas-pc): CAPABILITY has action: 1, code: 1, length 10
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [Z1DRQ-N6Z5F] 127.0.0.1(donatas-pc): Dynamic Capability MultiProtocol Extensions afi/safi invalid (bad-value/unicast)
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [JTVED-VGTQQ] 127.0.0.1(donatas-pc): CAPABILITY has action: 1, code: 1, length 10
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [Z1DRQ-N6Z5F] 127.0.0.1(donatas-pc): Dynamic Capability MultiProtocol Extensions afi/safi invalid (bad-value/unicast)
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [JTVED-VGTQQ] 127.0.0.1(donatas-pc): CAPABILITY has action: 1, code: 1, length 10
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [Z1DRQ-N6Z5F] 127.0.0.1(donatas-pc): Dynamic Capability MultiProtocol Extensions afi/safi invalid (bad-value/unicast)
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [JTVED-VGTQQ] 127.0.0.1(donatas-pc): CAPABILITY has action: 1, code: 1, length 10
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [Z1DRQ-N6Z5F] 127.0.0.1(donatas-pc): Dynamic Capability MultiProtocol Extensions afi/safi invalid (bad-value/unicast)
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [JTVED-VGTQQ] 127.0.0.1(donatas-pc): CAPABILITY has action: 1, code: 1, length 10
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [Z1DRQ-N6Z5F] 127.0.0.1(donatas-pc): Dynamic Capability MultiProtocol Extensions afi/safi invalid (bad-value/unicast)
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [JTVED-VGTQQ] 127.0.0.1(donatas-pc): CAPABILITY has action: 1, code: 1, length 10
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [Z1DRQ-N6Z5F] 127.0.0.1(donatas-pc): Dynamic Capability MultiProtocol Extensions afi/safi invalid (bad-value/unicast)
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [JTVED-VGTQQ] 127.0.0.1(donatas-pc): CAPABILITY has action: 1, code: 1, length 10
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [Z1DRQ-N6Z5F] 127.0.0.1(donatas-pc): Dynamic Capability MultiProtocol Extensions afi/safi invalid (bad-value/unicast)
Mar 29 11:14:54 donatas-laptop bgpd[347675]: [JTVED-VGTQQ] 127.0.0.1(donatas-pc): CAPABILITY has action: 1, code: 1, length 10
```

Reported-by: Iggy Frankovic <iggyfran@amazon.com>
Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>