bgpd: Fix crash for `show ip bgp vrf all all neighbors 192.168.0.1 ...`

bgpd: Fix crash for `show ip bgp vrf all all neighbors 192.168.0.1 ...`

When `all` is specified BGP pointer is always NULL, we need to iterate over
all instances separately.

```
Received signal 11 at 1648199394 (si_addr 0x30, PC 0x562e96597090); aborting...
 /usr/local/lib/libfrr.so.0(zlog_backtrace_sigsafe+0x5e) [0x7f378a57ff6e]
 /usr/local/lib/libfrr.so.0(zlog_signal+0xe6) [0x7f378a580146]
 /usr/local/lib/libfrr.so.0(+0xcd4c2) [0x7f378a5aa4c2]
 /lib/x86_64-linux-gnu/libpthread.so.0(+0x14140) [0x7f378a33e140]
 /usr/lib/frr/bgpd(bgp_afi_safi_peer_exists+0) [0x562e96597090]
 /usr/lib/frr/bgpd(+0x15c3b8) [0x562e9654a3b8]
```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>