exposay: don't crash if a view goes away
exposay: don't crash if a view goes away

When a view was destroyed while we were on exposay, we didn't
remove it from the list of views, and so when leaving exposay
we were trying to animate (and sometimes activate) a
non-existent view, causing a crash.

Signed-off-by: Emilio Pozuelo Monfort <74af59c529c33a30889589bc73d19ca9a6fb0e6a@collabora.co.uk>

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