qjson: free str

Release the qstring allocated in qjson_new().

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

migration/qjson.c

@@ -109,5 +109,6 @@ void qjson_finish(QJSON *json)
 
 void qjson_destroy(QJSON *json)
 {
+    QDECREF(json->str);
     g_free(json);
 }