shell: don't segfault when pressing super+k when no surface is focused
shell: don't segfault when pressing super+k when no surface is focused

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