bgpd: Do not crash if bgp argument is NULL for bgp_table_stats()

bgpd: Do not crash if bgp argument is NULL for bgp_table_stats()

```
(gdb) bt
0  0x00007f45a6f0a781 in raise () from /lib/x86_64-linux-gnu/libc.so.6
1  0x00007f45a6ef455b in abort () from /lib/x86_64-linux-gnu/libc.so.6
2  0x00007f45a7781920 in core_handler (signo=11, siginfo=0x7fffac7b84b0, context=<optimized out>) at lib/sigevent.c:228
3  <signal handler called>
4  0x000055a4133c0f32 in bgp_table_stats (vty=vty@entry=0x55a415acb240, bgp=0x0, afi=AFI_IP, safi=SAFI_UNICAST, json_array=json_array@entry=0x0) at bgpd/bgp_route.c:11412
5  0x000055a4133c13fb in show_ip_bgp_afi_safi_statistics (self=<optimized out>, vty=0x55a415acb240, argc=6, argv=<optimized out>) at bgpd/bgp_route.c:10749
6  0x00007f45a773917d in cmd_execute_command_real (vline=vline@entry=0x55a415ab7e10, vty=vty@entry=0x55a415acb240, cmd=cmd@entry=0x0, filter=FILTER_RELAXED)
    at lib/command.c:909
7  0x00007f45a773afdf in cmd_execute_command (vline=vline@entry=0x55a415ab7e10, vty=vty@entry=0x55a415acb240, cmd=0x0, vtysh=vtysh@entry=0) at lib/command.c:968
8  0x00007f45a773b135 in cmd_execute (vty=vty@entry=0x55a415acb240, cmd=cmd@entry=0x55a415ace950 "show ip bgp vrf all statistics", matched=matched@entry=0x0,
    vtysh=vtysh@entry=0) at lib/command.c:1122
9  0x00007f45a7794d62 in vty_command (vty=vty@entry=0x55a415acb240, buf=0x55a415ace950 "show ip bgp vrf all statistics") at lib/vty.c:526
10 0x00007f45a7794fb6 in vty_execute (vty=vty@entry=0x55a415acb240) at lib/vty.c:1293
11 0x00007f45a7797804 in vtysh_read (thread=<optimized out>) at lib/vty.c:2126
12 0x00007f45a778f641 in thread_call (thread=thread@entry=0x7fffac7bb040) at lib/thread.c:1550
13 0x00007f45a775b6d8 in frr_run (master=0x55a415542820) at lib/libfrr.c:1098
14 0x000055a4133815d6 in main (argc=10, argv=0x7fffac7bb2a8) at bgpd/bgp_main.c:509
```

"show ip bgp vrf all statistics" should show statistics for all VRFs if "all"
is specified.

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>