bgpd: Make sure we have enough data to read two bytes when validating…

… AIGP

Found when fuzzing:

```
==3470861==ERROR: AddressSanitizer: heap-buffer-overflow on address 0xffff77801ef7 at pc 0xaaaaba7b3dbc bp 0xffffcff0e760 sp 0xffffcff0df50
READ of size 2 at 0xffff77801ef7 thread T0
    0 0xaaaaba7b3db8 in __asan_memcpy (/home/ubuntu/frr_8_5_2/frr_8_5_2_fuzz_clang/bgpd/bgpd+0x363db8) (BuildId: cc710a2356e31c7f4e4a17595b54de82145a6e21)
    1 0xaaaaba81a8ac in ptr_get_be16 /home/ubuntu/frr_8_5_2/frr_8_5_2_fuzz_clang/./lib/stream.h:399:2
    2 0xaaaaba819f2c in bgp_attr_aigp_valid /home/ubuntu/frr_8_5_2/frr_8_5_2_fuzz_clang/bgpd/bgp_attr.c:504:3
    3 0xaaaaba808c20 in bgp_attr_aigp /home/ubuntu/frr_8_5_2/frr_8_5_2_fuzz_clang/bgpd/bgp_attr.c:3275:7
    4 0xaaaaba7ff4e0 in bgp_attr_parse /home/ubuntu/frr_8_5_2/frr_8_5_2_fuzz_clang/bgpd/bgp_attr.c:3678:10
```

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

bgpd/bgp_attr.c

@@ -502,6 +502,7 @@ static bool bgp_attr_aigp_valid(uint8_t *pnt, int length)
 	uint8_t *data = pnt;
 	uint8_t tlv_type;
 	uint16_t tlv_length;
+	uint8_t *end = data + length;
 
 	if (length < 3) {
 		zlog_err("Bad AIGP attribute length (MUST be minimum 3): %u",
@@ -510,7 +511,13 @@ static bool bgp_attr_aigp_valid(uint8_t *pnt, int length)
 	}
 
 	while (length) {
+		size_t data_len = end - data;
+
 		tlv_type = *data;
+
+		if (data_len - 1 < 2)
+			return false;
+
 		ptr_get_be16(data + 1, &tlv_length);
 		(void)data;