bgpd: Check against extended community unit size for link bandwidth

bgpd: Check against extended community unit size for link bandwidth

If we receive a malformed packets, this could lead ptr_get_be64() reading
the packets more than needed (heap overflow).

```
Using host libthread_db library "/lib/aarch64-linux-gnu/libthread_db.so.1".
    0 0xaaaaaadf86ec in __asan_memcpy (/home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/.libs/bgpd+0x3586ec) (BuildId: 78123cd26ada92b8b59fc0d74d292ba70c9d2e01)
    1 0xaaaaaaeb60fc in ptr_get_be64 /home/ubuntu/frr-public/frr_public_private-libfuzzer/./lib/stream.h:377:2
    2 0xaaaaaaeb5b90 in ecommunity_linkbw_present /home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/bgp_ecommunity.c:1895:10
    3 0xaaaaaae50f30 in bgp_attr_ext_communities /home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/bgp_attr.c:2639:8
    4 0xaaaaaae49d58 in bgp_attr_parse /home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/bgp_attr.c:3776:10
    5 0xaaaaab063260 in bgp_update_receive /home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/bgp_packet.c:2371:20
    6 0xaaaaab05df00 in bgp_process_packet /home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/bgp_packet.c:4063:11
    7 0xaaaaaae36110 in LLVMFuzzerTestOneInput /home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/bgp_main.c:582:3
```

This is triggered when receiving such a packet (malformed):

```
(gdb) bt
0  ecommunity_linkbw_present (ecom=0x555556287990, bw=bw@entry=0x7fffffffda68)
    at bgpd/bgp_ecommunity.c:1802
1  0x000055555564fcac in bgp_attr_ext_communities (args=0x7fffffffd840) at bgpd/bgp_attr.c:2619
2  bgp_attr_parse (peer=peer@entry=0x55555628cdf0, attr=attr@entry=0x7fffffffd960, size=size@entry=20,
    mp_update=mp_update@entry=0x7fffffffd940, mp_withdraw=mp_withdraw@entry=0x7fffffffd950)
    at bgpd/bgp_attr.c:3755
3  0x00005555556aa655 in bgp_update_receive (connection=connection@entry=0x5555562aa030,
    peer=peer@entry=0x55555628cdf0, size=size@entry=41) at bgpd/bgp_packet.c:2324
4  0x00005555556afab7 in bgp_process_packet (thread=<optimized out>) at bgpd/bgp_packet.c:3897
5  0x00007ffff7ac2f73 in event_call (thread=thread@entry=0x7fffffffdc70) at lib/event.c:2011
6  0x00007ffff7a6fb90 in frr_run (master=0x555555bc7c90) at lib/libfrr.c:1212
7  0x00005555556457e1 in main (argc=<optimized out>, argv=<optimized out>) at bgpd/bgp_main.c:543
(gdb) p *ecom
$1 = {refcnt = 1, unit_size = 8 '\b', disable_ieee_floating = false, size = 2, val = 0x555556282150 "",
  str = 0x5555562a9c30 "UNK:0, 255 UNK:2, 6"}
```

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