Fix possible memory corruption (FS#734)

Fix possible memory corruption (FS#734)

The memory referred to by the reply argument of
property_update_wm_protocols is automatically free'd
by xcb later on, so it is not safe to simply use the
value of reply in our own data structures. If we did
this, future calls to xcb_get_wm_protocols_reply_wipe
free the data which has already been free'd by xcb,
causing a double-free and corrupting the heap. In
addition, it isn't safe to use free'd memory as if
it is still allocated. Instead, duplicate the data
referred to by reply and use the duplicate instead.

It seems to me as if the duplication should actually
be done in xcb_get_wm_protocols_from_reply, but I'm
not really sure. If that is the case, this is simply
a work-around until xcb can be fixed.

Signed-off-by: Ari Entlich <atrigent@ccs.neu.edu>
Signed-off-by: Julien Danjou <julien@danjou.info>