bgpd: fix static analyzer issues around bgp pointer

bgpd: fix static analyzer issues around bgp pointer

Some static analyzer issues can be observed in BGP code:

> In file included from ./lib/zebra.h:13,
>                  from lib/event.c:8:
> ./lib/compiler.h:222:26: note: '#pragma message: Remove `clear thread cpu` command'
>   222 | #define CPP_NOTICE(text) _Pragma(CPP_STR(message text))
>       |                          ^~~~~~~
> lib/event.c:433:1: note: in expansion of macro 'CPP_NOTICE'
>   433 | CPP_NOTICE("Remove `clear thread cpu` command")
>       | ^~~~~~~~~~
> bgpd/bgp_vty.c:1592:5: warning: Access to field 'as_pretty' results in a dereference of a null pointer (loaded from variable 'bgp') [core.NullDereference]
> 1592 |                                 bgp->as_pretty);
>       |                                 ^~~~~~~~~~~~~~
> bgpd/bgp_vty.c:1599:5: warning: Access to field 'as_pretty' results in a dereference of a null pointer (loaded from variable 'bgp') [core.NullDereference]
> 1599 |                                 bgp->as_pretty);
>       |                                 ^~~~~~~~~~~~~~
> bgpd/bgp_vty.c:1612:7: warning: Access to field 'flags' results in a dereference of a null pointer (loaded from variable 'bgp') [core.NullDereference]
> 1612 |                     IS_BGP_INSTANCE_HIDDEN(bgp)) {
>       |                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~
> ./bgpd/bgpd.h:2906:3: note: expanded from macro 'IS_BGP_INSTANCE_HIDDEN'
> 2906 |         (CHECK_FLAG(_bgp->flags, BGP_FLAG_INSTANCE_HIDDEN) &&                  \
>       |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ./lib/zebra.h:274:31: note: expanded from macro 'CHECK_FLAG'
>   274 | #define CHECK_FLAG(V,F)      ((V) & (F))
>       |                               ^~~
> bgpd/bgp_vty.c:1614:4: warning: Access to field 'flags' results in a dereference of a null pointer (loaded from variable 'bgp') [core.NullDereference]
> 1614 |                         UNSET_FLAG(bgp->flags, BGP_FLAG_INSTANCE_HIDDEN);
>       |                         ^          ~~~
> ./lib/zebra.h:276:34: note: expanded from macro 'UNSET_FLAG'
>   276 | #define UNSET_FLAG(V,F)      (V) &= ~(F)
>       |                               ~  ^
> 4 warnings generated.
> Static Analysis warning summary compared to base:

Fix those issues by protecting the bgp pointer.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>