bgpd: Fix crash when deleting the SRv6 locator

bgpd: Fix crash when deleting the SRv6 locator

When BGP receives a `SRV6_LOCATOR_DEL` from zebra, it invokes
`bgp_zebra_process_srv6_locator_delete` to process the message.

`bgp_zebra_process_srv6_locator_delete` obtains a pointer to the default
BGP instance and then dereferences this pointer.

If the default BGP instance is not ready / not configured yet, this
pointer this pointer is `NULL` and dereferencing it causes BGP to crash.

This commit fix the issue by adding a a check to verify if the pointer
is `NULL` and returning early if it is.

Signed-off-by: Carmine Scarpitta <cscarpit@cisco.com>