bsc_api: Clear lchan->conn on lchan_release

Set conn to NULL before starting the release procedure, remove
code to check for a lchan->conn as it can not happen. If there
are any memleaks we will notice them.
Detach the lchan->conn from the channel before detaching.
diff --git a/openbsc/src/bsc_api.c b/openbsc/src/bsc_api.c
index 59b6af8..f1f7a62 100644
--- a/openbsc/src/bsc_api.c
+++ b/openbsc/src/bsc_api.c
@@ -368,10 +368,8 @@
 	if (conn->ho_lchan)
 		bsc_clear_handover(conn);
 
-	if (conn->lchan) {
+	if (conn->lchan)
 		lchan_release(conn->lchan, 1, 0);
-		conn->lchan->conn = NULL;
-	}
 
 	conn->lchan = NULL;
 	conn->ho_lchan = NULL;