bgpd: Check if we have really enough data before doing memcpy for FQD…

…N capability

We advance data pointer (data++), but we do memcpy() with the length that is 1-byte
over, which is technically heap overflow.

```
==411461==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x50600011da1a at pc 0xc4f45a9786f0 bp 0xffffed1e2740 sp 0xffffed1e1f30
READ of size 4 at 0x50600011da1a thread T0
    0 0xc4f45a9786ec in __asan_memcpy (/home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/.libs/bgpd+0x3586ec) (BuildId: e794c5f796eee20c8973d7efb9bf5735e54d44cd)
    1 0xc4f45abf15f8 in bgp_dynamic_capability_fqdn /home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/bgp_packet.c:3457:4
    2 0xc4f45abdd408 in bgp_capability_msg_parse /home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/bgp_packet.c:3911:4
    3 0xc4f45abdbeb4 in bgp_capability_receive /home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/bgp_packet.c:3980:9
    4 0xc4f45abde2cc in bgp_process_packet /home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/bgp_packet.c:4109:11
    5 0xc4f45a9b6110 in LLVMFuzzerTestOneInput /home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/bgp_main.c:582:3
```

Found by fuzzing.

Reported-by: Iggy Frankovic <[email protected]>
Signed-off-by: Donatas Abraitis <[email protected]>

bgpd/bgp_packet.c

@@ -3437,7 +3437,7 @@ static void bgp_dynamic_capability_fqdn(uint8_t *pnt, int action,
 		}
 
 		len = *data;
-		if (data + len > end) {
+		if (data + len + 1 > end) {
 			zlog_err("%pBP: Received invalid FQDN capability length (host name) %d",
 				 peer, hdr->length);
 			return;
@@ -3468,7 +3468,7 @@ static void bgp_dynamic_capability_fqdn(uint8_t *pnt, int action,
 
 		/* domainname */
 		len = *data;
-		if (data + len > end) {
+		if (data + len + 1 > end) {
 			zlog_err("%pBP: Received invalid FQDN capability length (domain name) %d",
 				 peer, len);
 			return;