input/keyboard: reset seat keyboard on destroy

If a sway keyboard is being destroyed, then the keyboard is being
removed from a seat. If the associated wlr_keyboard is the currently
set keyboard for the wlr_seat, then we need to reset the wlr_seat's
keyboard to NULL so it doesn't reference an invalid device for the seat.
The next configured keyboard from the seat or the next keyboard from
that seat that has an event will then become the seat keyboard.
Similarly, this needs to be done for a wlr_keyboard_group's keyboard
when the wlr_keyboard_group is being destroyed.

sway/input/keyboard.c

@@ -675,9 +675,7 @@ static void sway_keyboard_group_remove(struct sway_keyboard *keyboard) {
 		struct sway_keyboard_group *sway_group = wlr_group->data;
 		wlr_group->data = NULL;
 		wl_list_remove(&sway_group->link);
-		wl_list_remove(&sway_group->keyboard_key.link);
-		wl_list_remove(&sway_group->keyboard_modifiers.link);
-		free(sway_group->seat_device->keyboard);
+		sway_keyboard_destroy(sway_group->seat_device->keyboard);
 		free(sway_group->seat_device->input_device);
 		free(sway_group->seat_device);
 		free(sway_group);
@@ -921,6 +919,11 @@ void sway_keyboard_destroy(struct sway_keyboard *keyboard) {
 	if (keyboard->seat_device->input_device->wlr_device->keyboard->group) {
 		sway_keyboard_group_remove(keyboard);
 	}
+	struct wlr_seat *wlr_seat = keyboard->seat_device->sway_seat->wlr_seat;
+	struct sway_input_device *device = keyboard->seat_device->input_device;
+	if (wlr_seat_get_keyboard(wlr_seat) == device->wlr_device->keyboard) {
+		wlr_seat_set_keyboard(wlr_seat, NULL);
+	}
 	if (keyboard->keymap) {
 		xkb_keymap_unref(keyboard->keymap);
 	}