bgpd: fix res validity in rpki_create_socket

bgpd: fix res validity in rpki_create_socket

Fix coverity scanner issue 1575912 where res pointer is supposed to
valid in:

> socket = vrf_socket(res->ai_family, ...)

but is checked for validity a few lines later.

Note that vrf_getaddrinfo returns an error code if getaddrinfo() fails
to allocate res and in this case, rpki_create_socket() returns.

Fixes: a951752 ("bgpd: create cache server socket in vrf")
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>