bgpd: fix duplicate BGP instance created with unified config

bgpd: fix duplicate BGP instance created with unified config

When running the bgp_evpn_rt5 setup with unified config, memory leak
about a non deleted BGP instance happens.

> root@ubuntu2204hwe:~/frr/tests/topotests/bgp_evpn_rt5# cat /tmp/topotests/bgp_evpn_rt5.test_bgp_evpn/r1.asan.bgpd.1164105
>
> =================================================================
> ==1164105==ERROR: LeakSanitizer: detected memory leaks
>
> Indirect leak of 12496 byte(s) in 1 object(s) allocated from:
>     #0 0x7f358eeb4a57 in __interceptor_calloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:154
>     #1 0x7f358e877233 in qcalloc lib/memory.c:106
>     #2 0x55d06c95680a in bgp_create bgpd/bgpd.c:3405
>     #3 0x55d06c95a7b3 in bgp_get bgpd/bgpd.c:3805
>     #4 0x55d06c87a9b5 in bgp_get_vty bgpd/bgp_vty.c:603
>     #5 0x55d06c68dc71 in bgp_evpn_local_l3vni_add bgpd/bgp_evpn.c:7032
>     #6 0x55d06c92989b in bgp_zebra_process_local_l3vni bgpd/bgp_zebra.c:3204
>     #7 0x7f358e9e3feb in zclient_read lib/zclient.c:4626
>     #8 0x7f358e98082d in event_call lib/event.c:1996
>     #9 0x7f358e848931 in frr_run lib/libfrr.c:1232
>     #10 0x55d06c60eae1 in main bgpd/bgp_main.c:557
>     #11 0x7f358e229d8f in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58

Actually, a BGP VRF Instance is created in auto mode when creating the
global BGP instance for the L3 VNI. And again, an other BGP VRF instance
is created. Fix this by ensuring that a non existing BGP instance is not
present. If it is present, and with auto mode or in hidden mode, then
override the AS value.

Fixes: f153b9a9b636 ("bgpd: Ignore auto created VRF BGP instances")
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>