desktop-shell: Don't process surfaces under destruction during output resize

An output resize may occur while a shell surface is in the process of
being destroyed, e.g., while the destruction fade-out animation is in
progress. In such cases although the shell surface exists, it's missing
the backing weston_desktop_surface, so ensure to check for that.

Fixes: #805
Fixes: eefd8ae2
Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>