desktop-shell: Don't activate child shsurf w/o a weston_desktop_surface

This was just some band-aid being added in desktop-shell which fixed it on the surface.

A better follow up happens in !738 (closed). Closing this one in favor of that.

Do not attempt to pass a child shsurf without having a valid weston_desktop_surface.

Fixes: #583 (closed)

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