wayland/cursor-role: Handle premature wl_buffer destruction

If a client would attach a buffer to a surface, commit, destroy the
buffer and then later set the surface as a cursor, there will be no
wl_buffer available to be used by the cursor role. Instead of
dereferencing the non-existing wl_buffer resource, handle this situation
by logging a warning and treating a prematurely destroyd wl_buffer as if
no buffer had been attached.

https://bugzilla.gnome.org/show_bug.cgi?id=770992