xwayland: set surface_id to 0 when client resource is found

xwayland: set surface_id to 0 when client resource is found

When Xwayland requests that a wl_surface be created and the X event is
handled before the wayland requests, a surface ID is stored to
window->surface_id and the window is added to the unpaired window list. When
weston_wm_create_surface is called, the window is removed from the list and
window->surface_id is set to zero. If window->surface_id is not zero when
weston_wm_window_destroy is called, the window is assumed to be in the
unpaired window list and wl_list_remove is called. If
weston_wm_window_handle_surface_id is called and the surface has already
been created, the window is not added to the unpaired window list, but
window->surface_id isn't set to zero. When the window is destroyed, removing
the window from the list is attempted anyway and a crash occurs.

This patch stores the surface ID in a temporary variable and only assigns it
to window->surface_id when the window is added to the unpaired window list.
Otherwise window->surface_id is set to zero to maintain its use as a flag
variable.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=80273
Signed-off-by: Tyler Veness <calcmogul@gmail.com>