update: Make --appstream work again

We need to handle opt_appstream before we try to parse the
given name as an app.

app/flatpak-builtins-update.c

@@ -212,15 +212,15 @@ flatpak_builtin_update (int           argc,
   if (opt_arch == NULL)
     opt_arch = (char *)flatpak_get_arch ();
 
-  if (!flatpak_split_partial_ref_arg (name, &opt_arch, &branch, error))
-    return FALSE;
-
   if (!opt_app && !opt_runtime)
     opt_app = opt_runtime = TRUE;
 
   if (opt_appstream)
     return update_appstream (dir, name, cancellable, error);
 
+  if (!flatpak_split_partial_ref_arg (name, &opt_arch, &branch, error))
+    return FALSE;
+
   if (opt_app)
     {
       g_auto(GStrv) refs = NULL;