guard seat_get_focus() NULL dereferences

[hack-heart]

seat_get_focus() can return NULL when the focus stack is empty. a few call sites don't check for this and just dereference the result, which crashes sway.

this happens when a client disconnects abruptly, leaving the seat with a stale has_focus flag or empty focus stack. whatever touches focus next dereferences NULL. the two confirmed crash sites were seat_set_workspace_focus and seat_set_focus_surface (coredumps below). seat_unfocus_unless_client has the same pattern so a check was added there too for safety.

Coredumps:

I cannot remember how i triggered this one:

#0  seat_send_unfocus ()
#1  seat_set_workspace_focus ()
#2  seat_set_focus ()
#3  wl_signal_emit_mutable ()
#4  container_begin_destroy ()
#5  view_unmap ()
#6  handle_unmap ()
#7  wl_signal_emit_mutable ()
#8  wlr_surface_unmap ()
#9  destroy_xdg_toplevel ()
#10 destroy_xdg_surface_role_object ()
#11 destroy_xdg_surface ()
#12 xdg_client_handle_resource_destroy ()
#13 wl_client_destroy ()

Mouse click on empty container (clicked wallpaper, likely was just after a wallpaper engine crash or something):

#0  seat_send_unfocus ()
#1  seat_set_workspace_focus ()
#2  seat_set_focus ()
#3  handle_button ()
#4  wl_signal_emit_mutable ()
#5  handle_pointer_button ()
#6  handle_libinput_readable ()
#7  wl_event_loop_dispatch ()

IPC workspace switch (with noctalia workspace widget):

#0  seat_send_unfocus ()
#1  seat_set_workspace_focus ()
#2  seat_set_focus ()
#3  workspace_switch ()
#4  cmd_workspace ()
#5  execute_command ()
#6  ipc_client_handle_command ()
#7  ipc_client_handle_readable ()
#8  wl_event_loop_dispatch ()

Layer surface teardown (restarted noctalia shell):

#0  seat_send_unfocus ()
#1  seat_set_focus_surface ()
#2  seat_set_focus_layer ()
#3  handle_node_destroy ()
#4  wl_signal_emit_mutable ()
#5  sway_scene_node_destroy ()
#6  layer_surface_destroy ()
#7  surface_handle_role_resource_destroy ()

i encountered this while using scroll and initially opened a pr against it dawsers/scroll#219

[emersion]

Shouldn't we set seat->has_focus to false when setting the focus to NULL?

[hack-heart]

just added that

[hack-heart]

@emersion did you get a chance to re-review?