osmux: Keep decoding osmux pkt if a batch contains an unknown CID

Receiving an unknown CID (for which we don't have an active conn) is
totally expected at the end of the call, were some messages from the
peer may be send for a while, specially if latency is high.
If this occurs, we simply drop that batch and keep decoding the osmux
message in order to keep delivering content to other circuits.

Related: SYS#5987
Change-Id: I315949853bdcc07bef15d2e8579029cc72c427cf

src/libosmo-mgcp/mgcp_osmux.c

@@ -443,7 +443,8 @@ static int osmux_read_fd_cb(struct osmo_fd *ofd, unsigned int what)
 			LOGP(DOSMUX, LOGL_DEBUG,
 			     "Cannot find a src conn for circuit_id=%d\n",
 			     osmuxh->circuit_id);
-			goto out;
+			rem = msg->len;
+			continue;
 		}
 		endp = conn_src->conn->endp;
 		mgcp_conn_watchdog_kick(conn_src->conn);