qemu_domain: Don't unref NULL hash table in qemuDomainRefreshStatsSchema()

The g_hash_table_unref() function does not accept NULL. Passing
NULL results in a glib warning being triggered. Check whether the
hash table is not NULL and unref it only then.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

src/qemu/qemu_domain.c

@@ -11923,7 +11923,8 @@ qemuDomainRefreshStatsSchema(virDomainObj *dom)
     if (!schema)
         return -1;
 
-    g_hash_table_unref(priv->statsSchema);
+    if (priv->statsSchema)
+        g_hash_table_unref(priv->statsSchema);
     priv->statsSchema = schema;
 
     return 0;