shell: validate create_focus_surface results
shell: validate create_focus_surface results

Signed-off-by: U. Artie Eoff <32aece0d55055227b44d04b7d0051cf3d04ee871@intel.com>

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