desktop-shell: Guard against invalid seats in get_shell_seat()

Starting with commit 'desktop-shell: Embed keyboard focus handle code when activating' we are iterating over all the seats when removing a weston_desktop_surface to be able to invalidate the active surface.

Upon compositor tear down the shell destroy might invalidate seats before removal of the weston_desktop_surface making the shell_seat itself invalid. This wasn't apparent at that time because we're not handling at that time removal of surfaces from layers.

Signed-off-by: Marius Vlad marius.vlad@collabora.com