metadata: return if meta_tree_refresh failed

G_UNLOCK is called twice if meta_tree_refresh fails and also concurrent
access is possible on cached_trees. However this isn't problem currently,
because meta_tree_lookup_by_name is always called with for_write=FALSE
(except for meta-set testing utility), so this if statement is never issued.

https://bugzilla.gnome.org/show_bug.cgi?id=598561