fix 'invalid tree path' exception in thumbnail view

Happens when switching quickly between archives: with the way thumbnails
are generated in another thread, and the update is deferred to the main
thread, we sometimes get the update callback called after we've already
stopped generating new thumbnails.

Thanks to Benoit Pierre for this patch.


git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@1034 ef8490bc-f317-4725-81ed-e95a9aca1bcf