x11/xprops: Plug a few memory leaks

Commits 6dbec6f8, 734402e1 and f041b35b introduced memory leaks by
switching to returning copies instead of the original buffers but
forgetting to free those original buffers.

Some error cases were also not freeing the ->prop buffer as they
should.

https://bugzilla.gnome.org/show_bug.cgi?id=642652