bgpd: Avoid use-after-free when doing `no router bgp` with auto created instances

bgpd: Avoid use-after-free when doing `no router bgp` with auto created instances

```
==1145965==ERROR: AddressSanitizer: heap-use-after-free on address 0x6030007159c0 at pc 0x55ade8d962d1 bp 0x7ffec4ce74c0 sp 0x7ffec4ce74b0
READ of size 8 at 0x6030007159c0 thread T0
    0 0x55ade8d962d0 in no_router_bgp bgpd/bgp_vty.c:1701
    1 0x7efe5aed19ed in cmd_execute_command_real lib/command.c:1002
    2 0x7efe5aed1da3 in cmd_execute_command lib/command.c:1061
    3 0x7efe5aed2303 in cmd_execute lib/command.c:1227
    4 0x7efe5af6c023 in vty_command lib/vty.c:616
    5 0x7efe5af6d2d2 in vty_execute lib/vty.c:1379
    6 0x7efe5af77df2 in vtysh_read lib/vty.c:2374
    7 0x7efe5af64c9b in event_call lib/event.c:1996
    8 0x7efe5af03887 in frr_run lib/libfrr.c:1232
    9 0x55ade8cd9850 in main bgpd/bgp_main.c:555
    10 0x7efe5aa29d8f in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
    11 0x7efe5aa29e3f in __libc_start_main_impl ../csu/libc-start.c:392
    12 0x55ade8cdc314 in _start (/usr/lib/frr/bgpd+0x16f314)
```

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