abis_rsl_rx_dchan(): guard against lchan_lookup() returning NULL

Found this by coincidence, no actual failure case was observed.

lchan_lookup() does have a return NULL code path, so we should not blindly use
its returned pointer.

Change-Id: I34ce126d36420b8194c88c0faa865294334a6658

openbsc/src/libbsc/abis_rsl.c

@@ -1456,6 +1456,8 @@ static int abis_rsl_rx_dchan(struct msgb *msg)
 
 	msg->lchan = lchan_lookup(sign_link->trx, rslh->chan_nr,
 				  "Abis RSL rx DCHAN: ");
+	if (!msg->lchan)
+		return -1;
 	ts_name = gsm_lchan_name(msg->lchan);
 
 	switch (rslh->c.msg_type) {