compositor: use wl_list_for_each_safe when destroying output
compositor: use wl_list_for_each_safe when destroying output

Was causing an invalid read when the output is in fact destroyed. That's only
visible (segfault on my machine) on drm compositor because it's the only
backend trying to finish correct the compositor.

Signed-off-by: Tiago Vignatti <f080760b2054d506aacc8dee97f8804c19fbc3aa@intel.com>

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