MT#55283 Stop trickle updates without a room

Don't proceed with the trickle updates in
case the required room, hence a call
hasn't been found.

Fixes Coverity Scan defect:

*** CID 1600057:  Null pointer dereferences  (NULL_RETURNS)
/daemon/janus.c: 1679 in janus_trickle()
1673            // ufrag can be given in-line or separately
1674            sp->ice_ufrag = cand->ufrag;
1675            if (!sp->ice_ufrag.len && ufrag)
1676                    bencode_strdup_str(&ngbuf->buffer, &sp->ice_ufrag, ufrag);
1677
1678            // finally do the update
>>>     CID 1600057:  Null pointer dereferences  (NULL_RETURNS)
>>>     Dereferencing a pointer that might be "NULL" "call" when calling "trickle_ice_update".
1679            trickle_ice_update(ngbuf, call, &flags, &streams);
1680
1681            return NULL;
1682     }
1683
1684

Change-Id: Ib2e293c2f99e914e3d02fe43d08160ec30892ae4

daemon/janus.c

@@ -1616,9 +1616,15 @@ static const char *janus_trickle(JsonReader *reader, struct janus_session *sessi
 		call_id = janus_call_id(handle->room);
 
 		struct janus_room *room = t_hash_table_lookup(janus_rooms, &handle->room);
-
-		if (room)
-			call = call_get(&room->call_id);
+		if (!room) {
+			*retcode = 426;
+			return "No such room";
+		}
+		call = call_get(&room->call_id);
+		if (!call) {
+			*retcode = 426;
+			return "No such room";
+		}
 	}
 
 	// set up "streams" structures to use an trickle ICE update. these must be