zebra: fix possible null dereference

zebra: fix possible null dereference

Don't attempt to dereference `ifp` directly if it might be null: there
is a check right before this usage: `ifp ? ifp->info : NULL`.

In this context it should be safe to assume `ifp` is not NULL because
the only caller of this function checks that for this `ifindex`. For
consistency we'll check for null anyway in case this ever changes (and
with this the coverity scan warning gets silenced).

Found by Coverity Scan (CID 1519776)

Signed-off-by: Rafael Zalamena <rzalamena@opensourcerouting.org>