rtp_relay: fix leg dialog detection in local_route

When both dialog and b2b_logic modules are used with rtp_relay, we shoud
be prepared for a non-existing dialog in requests.

Reported by @devoxy1 on GitHub in ticket #2952

(cherry picked from commit 627e52a)

modules/rtp_relay/rtp_relay_ctx.c

@@ -464,8 +464,7 @@ static struct rtp_relay_leg *rtp_relay_get_peer_leg_ctx(struct rtp_relay_ctx *ct
 	struct rtp_relay_leg *leg;
 	struct b2b_entity_info_t info;
 
-	if (rtp_relay_dlg_ctx_idx != -1) {
-		dlg = rtp_relay_dlg.get_dlg();
+	if (rtp_relay_dlg_ctx_idx != -1 && (dlg = rtp_relay_dlg.get_dlg()) != NULL) {
 		if (rtp_relay_dlg.get_direction() == DLG_DIR_UPSTREAM) {
 			tag = &dlg->legs[DLG_CALLER_LEG].tag;
 		} else {