xwm: do not crash when activating a NULL surface
xwm: do not crash when activating a NULL surface

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