bgpd: Do not process NLRIs if the attribute length is zero

```
3  0x00007f423aa42476 in __GI_raise (sig=sig@entry=11) at ../sysdeps/posix/raise.c:26
4  0x00007f423aef9740 in core_handler (signo=11, siginfo=0x7fffc414deb0, context=<optimized out>) at lib/sigevent.c:246
5  <signal handler called>
6  0x0000564dea2fc71e in route_set_aspath_prepend (rule=0x564debd66d50, prefix=0x7fffc414ea30, object=0x7fffc414e400)
    at bgpd/bgp_routemap.c:2258
7  0x00007f423aeec7e0 in route_map_apply_ext (map=<optimized out>, prefix=prefix@entry=0x7fffc414ea30,
    match_object=match_object@entry=0x7fffc414e400, set_object=set_object@entry=0x7fffc414e400, pref=pref@entry=0x0) at lib/routemap.c:2690
8  0x0000564dea2d277e in bgp_input_modifier (peer=peer@entry=0x7f4238f59010, p=p@entry=0x7fffc414ea30, attr=attr@entry=0x7fffc414e770,
    afi=afi@entry=AFI_IP, safi=safi@entry=SAFI_UNICAST, rmap_name=rmap_name@entry=0x0, label=0x0, num_labels=0, dest=0x564debdd5130)
    at bgpd/bgp_route.c:1772
9  0x0000564dea2df762 in bgp_update (peer=peer@entry=0x7f4238f59010, p=p@entry=0x7fffc414ea30, addpath_id=addpath_id@entry=0,
    attr=0x7fffc414eb50, afi=afi@entry=AFI_IP, safi=<optimized out>, safi@entry=SAFI_UNICAST, type=9, sub_type=0, prd=0x0, label=0x0,
    num_labels=0, soft_reconfig=0, evpn=0x0) at bgpd/bgp_route.c:4374
10 0x0000564dea2e2047 in bgp_nlri_parse_ip (peer=0x7f4238f59010, attr=attr@entry=0x7fffc414eb50, packet=0x7fffc414eaf0)
    at bgpd/bgp_route.c:6249
11 0x0000564dea2c5a58 in bgp_nlri_parse (peer=peer@entry=0x7f4238f59010, attr=attr@entry=0x7fffc414eb50,
    packet=packet@entry=0x7fffc414eaf0, mp_withdraw=mp_withdraw@entry=false) at bgpd/bgp_packet.c:339
12 0x0000564dea2c5d66 in bgp_update_receive (peer=peer@entry=0x7f4238f59010, size=size@entry=109) at bgpd/bgp_packet.c:2024
13 0x0000564dea2c901d in bgp_process_packet (thread=<optimized out>) at bgpd/bgp_packet.c:2933
14 0x00007f423af0bf71 in event_call (thread=thread@entry=0x7fffc414ee40) at lib/event.c:1995
15 0x00007f423aebb198 in frr_run (master=0x564deb73c670) at lib/libfrr.c:1213
16 0x0000564dea261b83 in main (argc=<optimized out>, argv=<optimized out>) at bgpd/bgp_main.c:505
```

With the configuration:

```
frr version 9.1-dev-MyOwnFRRVersion
frr defaults traditional
hostname ip-172-31-13-140
log file /tmp/debug.log
log syslog
service integrated-vtysh-config
!
debug bgp keepalives
debug bgp neighbor-events
debug bgp updates in
debug bgp updates out
!
router bgp 100
 bgp router-id 9.9.9.9
 no bgp ebgp-requires-policy
 bgp bestpath aigp
 neighbor 172.31.2.47 remote-as 200
 !
 address-family ipv4 unicast
  neighbor 172.31.2.47 default-originate
  neighbor 172.31.2.47 route-map RM_IN in
 exit-address-family
exit
!
route-map RM_IN permit 10
 set as-path prepend 200
exit
!
```

The issue is that we try to process NLRIs even if the attribute length is 0.

Later bgp_update() will handle route-maps and a crash occurs because all the
attributes are NULL, including aspath, where we dereference.

According to the RFC 4271:

A value of 0 indicates that neither the Network Layer
         Reachability Information field nor the Path Attribute field is
         present in this UPDATE message.

But with a fuzzed UPDATE message this can be faked. I think it's reasonable
to skip processing NLRIs if both update_len and attribute_len are 0.

Reported-by: Iggy Frankovic <iggyfran@amazon.com>
Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>
(cherry picked from commit 28ccc24)

bgpd/bgp_packet.c

@@ -1983,7 +1983,7 @@ static int bgp_update_receive(struct peer *peer, bgp_size_t size)
 	/* Network Layer Reachability Information. */
 	update_len = end - stream_pnt(s);
 
-	if (update_len) {
+	if (update_len && attribute_len) {
 		/* Set NLRI portion to structure. */
 		nlris[NLRI_UPDATE].afi = AFI_IP;
 		nlris[NLRI_UPDATE].safi = SAFI_UNICAST;