tree data BUGFIX added return value for unlink

Added return value for functions lyd_unlink_siblings() and
lyd_unlink_tree(), which causes ABI breaks.
diff --git a/src/plugins_exts/schema_mount.c b/src/plugins_exts/schema_mount.c
index b349921..e812053 100644
--- a/src/plugins_exts/schema_mount.c
+++ b/src/plugins_exts/schema_mount.c
@@ -940,7 +940,7 @@
     }
 
     /* create accessible tree, remove LYD_EXT to not call this callback recursively */
-    lyd_unlink_siblings(sibling);
+    LY_CHECK_GOTO(lyd_unlink_siblings(sibling), cleanup);
     LY_LIST_FOR(sibling, iter) {
         iter->flags &= ~LYD_EXT;
     }