bgpd: Fix crash for `show ip bgp vrf all all`

bgpd: Fix crash for `show ip bgp vrf all all`

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

```
BGP[170822]: Received signal 11 at 1648199394 (si_addr 0x30, PC 0x562e96597090); aborting...
BGP[170822]: /usr/local/lib/libfrr.so.0(zlog_backtrace_sigsafe+0x5e) [0x7f378a57ff6e]
BGP[170822]: /usr/local/lib/libfrr.so.0(zlog_signal+0xe6) [0x7f378a580146]
BGP[170822]: /usr/local/lib/libfrr.so.0(+0xcd4c2) [0x7f378a5aa4c2]
BGP[170822]: /lib/x86_64-linux-gnu/libpthread.so.0(+0x14140) [0x7f378a33e140]
BGP[170822]: /usr/lib/frr/bgpd(bgp_afi_safi_peer_exists+0) [0x562e96597090]
BGP[170822]: /usr/lib/frr/bgpd(+0x15c3b8) [0x562e9654a3b8]
BGP[170822]: /usr/local/lib/libfrr.so.0(+0x75a9e) [0x7f378a552a9e]
BGP[170822]: /usr/local/lib/libfrr.so.0(cmd_execute_command+0x5d) [0x7f378a552e2d]
BGP[170822]: /usr/local/lib/libfrr.so.0(cmd_execute+0xc0) [0x7f378a553070]
BGP[170822]: /usr/local/lib/libfrr.so.0(+0xe3697) [0x7f378a5c0697]
BGP[170822]: /usr/local/lib/libfrr.so.0(+0xe3db1) [0x7f378a5c0db1]
BGP[170822]: /usr/local/lib/libfrr.so.0(+0xe6c30) [0x7f378a5c3c30]
BGP[170822]: /usr/local/lib/libfrr.so.0(thread_call+0x73) [0x7f378a5bb743]
BGP[170822]: /usr/local/lib/libfrr.so.0(frr_run+0xd0) [0x7f378a578750]
BGP[170822]: /usr/lib/frr/bgpd(main+0x344) [0x562e964cf3f4]
BGP[170822]: /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xea) [0x7f378a18bd0a]
BGP[170822]: /usr/lib/frr/bgpd(_start+0x2a) [0x562e964d10ea]
```

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