layer-shell: don't give focus to unmapped layer surfaces

Focused layers are not cleared when destroyed, they are cleared on unmap.
Giving focus to an unmapped layer surface is (1) incorrect and (2) triggers a
use-after-free.

Closes: https://github.com/swaywm/sway/issues/4517

sway/desktop/layer_shell.c

@@ -200,7 +200,8 @@ void arrange_layers(struct sway_output *output) {
 	for (size_t i = 0; i < nlayers; ++i) {
 		wl_list_for_each_reverse(layer,
 				&output->layers[layers_above_shell[i]], link) {
-			if (layer->layer_surface->current.keyboard_interactive) {
+			if (layer->layer_surface->current.keyboard_interactive &&
+					layer->layer_surface->mapped) {
 				topmost = layer;
 				break;
 			}

sway/input/seat.c

@@ -1095,6 +1095,7 @@ void seat_set_focus_layer(struct sway_seat *seat,
 	} else if (!layer || seat->focused_layer == layer) {
 		return;
 	}
+	assert(layer->mapped);
 	seat_set_focus_surface(seat, layer->surface, true);
 	if (layer->layer >= ZWLR_LAYER_SHELL_V1_LAYER_TOP) {
 		seat->focused_layer = layer;