xwm: set the window's shsurf pointer to NULL on unmap handling
xwm: set the window's shsurf pointer to NULL on unmap handling

the unmap event will be followed by the deletion of the weston_surface,
so the shell_surface will also be deleted by the shell. Having removed
the surface_destroy_listener, the surface_destroy callback doesn't
get called, so reset the value of shsurf here.

krezovic/weston,kwm81/weston,xorgy/weston,Gnurou/weston,mchalupa/weston,Fantu/compositor-spice,Tarnyko/weston-xdg_surface_present,xorgy/weston,mchalupa/weston,Fantu/compositor-spice,Tarnyko/weston-xdg_surface_present,eyolfson/weston,jonnylamb/weston,jonnylamb/weston,kwm81/weston,krezovic/weston,udoprog/weston,giucam/weston,sir-murray/weston,Gnurou/weston,udoprog/weston,Fantu/compositor-spice,sir-murray/weston,giucam/weston,eyolfson/weston