clients/desktop-shell: avoid invalid sized background

clients/desktop-shell: avoid invalid sized background

If for some reason the desktop-shell plugin would configure a background
with an invalid size, just destroy the whole background and forget
about it for this wl_output.

A following patch will cause desktop-shell to configure 0x0 background
when it deems the background redundant.

Fortify weston-desktop-shell against not every output having a
background.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>