mshtml: Fix string leak on NamespaceError.

dlls/mshtml/htmlelem.c

@@ -4857,6 +4857,7 @@ static HRESULT WINAPI HTMLElement6_setAttributeNS(IHTMLElement6 *iface, VARIANT
         return hres;
     nsAString_GetData(&ns_str, &ns);
     if((!ns || !ns[0]) && wcschr(strAttributeName, ':')) {
+        nsAString_Finish(&ns_str);
         /* FIXME: Return NamespaceError */
         return E_FAIL;
     }