guard seat_get_focus() NULL dereferences

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.

cannot remember how this one was triggered:

```
seat_send_unfocus ()
seat_set_workspace_focus ()
seat_set_focus ()
wl_signal_emit_mutable ()
container_begin_destroy ()
view_unmap ()
handle_unmap ()
wl_signal_emit_mutable ()
wlr_surface_unmap ()
destroy_xdg_toplevel ()
destroy_xdg_surface_role_object ()
destroy_xdg_surface ()
xdg_client_handle_resource_destroy ()
wl_client_destroy ()
```

mouse click on empty container (clicked wallpaper, likely just after a
wallpaper engine crash):

```
seat_send_unfocus ()
seat_set_workspace_focus ()
seat_set_focus ()
handle_button ()
wl_signal_emit_mutable ()
handle_pointer_button ()
handle_libinput_readable ()
wl_event_loop_dispatch ()
```

IPC workspace switch (with noctalia workspace widget):

```
seat_send_unfocus ()
seat_set_workspace_focus ()
seat_set_focus ()
workspace_switch ()
cmd_workspace ()
execute_command ()
ipc_client_handle_command ()
ipc_client_handle_readable ()
wl_event_loop_dispatch ()
```

layer surface teardown (restarted noctalia shell):

```
seat_send_unfocus ()
seat_set_focus_surface ()
seat_set_focus_layer ()
handle_node_destroy ()
wl_signal_emit_mutable ()
sway_scene_node_destroy ()
layer_surface_destroy ()
surface_handle_role_resource_destroy ()
```

Author: hack-heart

sway/input/seat.c

@@ -1136,10 +1136,12 @@ static void seat_set_workspace_focus(struct sway_seat *seat, struct sway_node *n
 
 	if (node == NULL) {
 		// Close any popups on the old focus
-		if (node_is_view(last_focus)) {
-			view_close_popups(last_focus->sway_container->view);
+		if (last_focus) {
+			if (node_is_view(last_focus)) {
+				view_close_popups(last_focus->sway_container->view);
+			}
+			seat_send_unfocus(last_focus, seat);
 		}
-		seat_send_unfocus(last_focus, seat);
 		sway_input_method_relay_set_focus(&seat->im_relay, NULL);
 		seat->has_focus = false;
 		return;
@@ -1291,7 +1293,9 @@ void seat_set_focus_surface(struct sway_seat *seat,
 		struct wlr_surface *surface, bool unfocus) {
 	if (seat->has_focus && unfocus) {
 		struct sway_node *focus = seat_get_focus(seat);
-		seat_send_unfocus(focus, seat);
+		if (focus) {
+			seat_send_unfocus(focus, seat);
+		}
 		seat->has_focus = false;
 	}
 
@@ -1340,7 +1344,7 @@ void seat_unfocus_unless_client(struct sway_seat *seat, struct wl_client *client
 	}
 	if (seat->has_focus) {
 		struct sway_node *focus = seat_get_focus(seat);
-		if (node_is_view(focus) && wl_resource_get_client(
+		if (focus && node_is_view(focus) && wl_resource_get_client(
 					focus->sway_container->view->surface->resource) != client) {
 			seat_set_focus(seat, NULL);
 		}