gsm_04_11: Fix a crash... in case there is a already a connection

If there is a connection, return before paging... otherwise
we will delete the SMS twice.
diff --git a/openbsc/src/gsm_04_11.c b/openbsc/src/gsm_04_11.c
index 8a85933..4488f8d 100644
--- a/openbsc/src/gsm_04_11.c
+++ b/openbsc/src/gsm_04_11.c
@@ -1150,7 +1150,7 @@
 	lchan = lchan_for_subscr(subscr);
 	if (lchan) {
 		use_subscr_con(&lchan->conn);
-		gsm411_send_sms_lchan(&lchan->conn, sms);
+		return gsm411_send_sms_lchan(&lchan->conn, sms);
 	}
 
 	/* if not, we have to start paging */