qom: Don't make link NULL on object_property_set_link() failure

The error behavior of object_property_set_link() is dangerous.  It sets
the link property object to NULL if an error occurs.  A setter function
should either succeed or fail, it shouldn't leave the value NULL on
failure.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>

qom/object.c

@@ -1080,27 +1080,28 @@ static Object *object_resolve_link(Object *obj, const char *name,
 static void object_set_link_property(Object *obj, Visitor *v, void *opaque,
                                      const char *name, Error **errp)
 {
+    Error *local_err = NULL;
     Object **child = opaque;
-    Object *old_target;
-    char *path;
-
-    visit_type_str(v, &path, name, errp);
+    Object *old_target = *child;
+    Object *new_target = NULL;
+    char *path = NULL;
 
-    old_target = *child;
-    *child = NULL;
+    visit_type_str(v, &path, name, &local_err);
 
-    if (strcmp(path, "") != 0) {
-        Object *target;
-
-        target = object_resolve_link(obj, name, path, errp);
-        if (target) {
-            object_ref(target);
-            *child = target;
-        }
+    if (!local_err && strcmp(path, "") != 0) {
+        new_target = object_resolve_link(obj, name, path, &local_err);
     }
 
     g_free(path);
+    if (local_err) {
+        error_propagate(errp, local_err);
+        return;
+    }
 
+    if (new_target) {
+        object_ref(new_target);
+    }
+    *child = new_target;
     if (old_target != NULL) {
         object_unref(old_target);
     }