bsc: Fix possible crash when the lchan->conn alloc failed

Another null pointer dereference found by clang.
diff --git a/openbsc/src/bsc_api.c b/openbsc/src/bsc_api.c
index a9e5f71..87ab981 100644
--- a/openbsc/src/bsc_api.c
+++ b/openbsc/src/bsc_api.c
@@ -478,9 +478,12 @@
 	} else {
 		rc = BSC_API_CONN_POL_REJECT;
 		lchan->conn = subscr_con_allocate(msg->lchan);
+		if (!lchan->conn) {
+			lchan_release(lchan, 0, 0);
+			return -1;
+		}
 
-		if (lchan->conn)
-			rc = api->compl_l3(lchan->conn, msg, 0);
+		rc = api->compl_l3(lchan->conn, msg, 0);
 
 		if (rc != BSC_API_CONN_POL_ACCEPT) {
 			lchan->conn->lchan = NULL;