xwm: Fix icon surface ownership

xwm: Fix icon surface ownership

The cairo surface used for the icon must be completely given to the
frame as soon as said frame has been created.  To prevent both the
window and the frame from sharing ownership of the icon, we set
window->icon_surface back to NULL right after creating or changing the
frame, only keeping it there when no frame has been created yet.

Fixes https://lists.freedesktop.org/archives/wayland-devel/2018-January/036655.html
Reported-by: Derek Foreman <derekf@osg.samsung.com>
Tested-by: Derek Foreman <derekf@osg.samsung.com>
Signed-off-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>