Fix libvirtd crash when removing metadata

When trying to remove nonexistent metadata from XML, libvirt daemon
crashes due to dereferencing NULL pointer.

Resolves https://bugzilla.redhat.com/show_bug.cgi?id=1143955

src/util/virxml.c

@@ -974,6 +974,9 @@ virXMLFindChildNodeByNs(xmlNodePtr root,
 {
     xmlNodePtr next;
 
+    if (!root)
+        return NULL;
+
     for (next = root->children; next; next = next->next) {
         if (next->ns &&
             STREQ_NULLABLE((const char *) next->ns->href, uri))