shell: Fix not destroy black surface when fullscreen surface exit abnomally.
shell: Fix not destroy black surface when fullscreen surface exit abnomally.

When a client with fullscreen surface displayed was aborted by Ctrl-C, the
black surface still be there. Destroy the black surface in
destroy_shell_surface().

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