gtphob: check for missing result of rate_ctr_group_alloc()

In case the counter group allocation fails, we must handle this
gracefully and fail the allocation of the parent object, too.

The recent change (Id I7dad4a4d52fe05f6b990359841b4408df5990e21) seems
to have missed one instance, so let's follow-up.

Change-Id: I1ee9e3d26dcc18e7f979fd9a786162cbcc50942c
Related: OS#2361

src/gprs/gtphub.c

@@ -2708,6 +2708,10 @@ static struct gtphub_peer_port *gtphub_addr_add_port(struct gtphub_peer_addr *a,
 
 	pp->counters_io = rate_ctr_group_alloc(osmo_gtphub_ctx,
 					       &gtphub_ctrg_io_desc, 0);
+	if (!pp->counters_io) {
+		talloc_free(pp);
+		return NULL;
+	}
 
 	llist_add(&pp->entry, &a->ports);