frame_relay: prevent null pointer exception when talloc fails

Change-Id: Ib39682ad0c7f30c97303b3d0bb6240fddd23ec0b
diff --git a/src/gb/frame_relay.c b/src/gb/frame_relay.c
index ae6bbf5..0304882 100644
--- a/src/gb/frame_relay.c
+++ b/src/gb/frame_relay.c
@@ -849,6 +849,8 @@
 struct osmo_fr_network *osmo_fr_network_alloc(void *ctx)
 {
 	struct osmo_fr_network *net = talloc_zero(ctx, struct osmo_fr_network);
+	if (!net)
+		return NULL;
 
 	INIT_LLIST_HEAD(&net->links);
 	net->T_defs = fr_tdefs;