Fix possible memory leak in util/virxml.c

A "xmlstr" string may not be assigned into a "doc" pointer and it
could cause memory leak. To fix it if the "doc" pointer is NULL and
the "xmlstr" string is not assigned we should free it.

This has been found by coverity.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

src/util/virxml.c

@@ -1047,6 +1047,8 @@ virXMLExtractNamespaceXML(xmlNodePtr root,
 cleanup:
     if (doc)
         *doc = xmlstr;
+    else
+        VIR_FREE(xmlstr);
     xmlFreeNode(nodeCopy);
     return ret;
 }