shell: fix resume_desktop for zero clients
shell: fix resume_desktop for zero clients

Fix two bugs:

- if there are no backgrounds at all, the background pointer would have
  been bogus. Lead to a segfault.

- if the hidden_surface_list is empty, wl_list_insert_list() would
  corrupt the list. Lead to a hang in pick_surface().

Signed-off-by: Pekka Paalanen <6c0263d81461c956d504307112a4ce25eaacc3d1@gmail.com>

kwm81/weston,Fantu/compositor-spice,udoprog/weston,mchalupa/weston,sir-murray/weston,Gnurou/weston,xorgy/weston,xorgy/weston,jonnylamb/weston,eyolfson/weston,krezovic/weston,jonnylamb/weston,krezovic/weston,Gnurou/weston,kwm81/weston,mchalupa/weston,Fantu/compositor-spice,Tarnyko/weston-xdg_surface_present,Tarnyko/weston-xdg_surface_present,udoprog/weston,giucam/weston,eyolfson/weston,Fantu/compositor-spice,sir-murray/weston,giucam/weston