desktop-shell: fix shell_output_destroy_move_layer unused variable

desktop-shell: fix shell_output_destroy_move_layer unused variable

/home/pq/git/weston/desktop-shell/shell.c: In function ‘shell_output_destroy_move_layer’:
/home/pq/git/weston/desktop-shell/shell.c:4718:24: warning: unused variable ‘output’ [-Wunused-variable]
  struct weston_output *output = data;

Since the data pointer is not used for anything, decided to also set it
to NULL in the caller. This caused another variable to become unused.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Marius-Vlad <marius-cristian.vlad@nxp.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>