call_leg: local_bridge: Avoid null pointer access if CN-side not ready

This happens if for instance an HNBGW drops the RAB-AssignmentRequest
and does nothing with it.

call_leg.c:348:15: runtime error: member access within null pointer of type 'struct rtp_stream'

Related: OS#5401
Change-Id: I67d2d5b2dd3b367c34f929d63c056306ec001431
diff --git a/src/libmsc/call_leg.c b/src/libmsc/call_leg.c
index 6242a80..e890f75 100644
--- a/src/libmsc/call_leg.c
+++ b/src/libmsc/call_leg.c
@@ -344,6 +344,9 @@
 	}
 	codec = cl1->rtp[RTP_TO_RAN]->codec;
 
+	if (!cl1->rtp[RTP_TO_CN] || !cl2->rtp[RTP_TO_CN])
+		return -ENOTCONN;
+
 	call_leg_ensure_ci(cl1, RTP_TO_CN, call_id1, trans1,
 			   &codec, &cl2->rtp[RTP_TO_CN]->local);
 	call_leg_ensure_ci(cl2, RTP_TO_CN, call_id2, trans2,