wlr_output_cursor: assert renderer isn't NULL

When a wlr_output add to wlr_output_layout, wlr_cursor will create
wlr_buffer of xcursor for the wlr_output, this behavior depends
wlr_renderer of wlr_output, using assert to ensure renderer is
initialized before wlr_output_layout_add and wlr_cursor_set_buffer/
wlr_cursor_set_xcursor.

types/output/cursor.c

@@ -394,9 +394,7 @@ static bool output_cursor_attempt_hardware(struct wlr_output_cursor *cursor) {
 bool wlr_output_cursor_set_buffer(struct wlr_output_cursor *cursor,
 		struct wlr_buffer *buffer, int32_t hotspot_x, int32_t hotspot_y) {
 	struct wlr_renderer *renderer = cursor->output->renderer;
-	if (!renderer) {
+	assert(renderer != NULL);
-		return false;
-	}
 
 	struct wlr_texture *texture = NULL;
 	struct wlr_fbox src_box = {0};

types/wlr_cursor.c

@@ -509,9 +509,7 @@ static void cursor_output_cursor_update(struct wlr_cursor_output_cursor *output_
 
 	if (cur->state->buffer != NULL) {
 		struct wlr_renderer *renderer = output_cursor->output_cursor->output->renderer;
-		if (!renderer) {
+		assert(renderer != NULL);
-			return;
-		}
 
 		struct wlr_buffer *buffer = cur->state->buffer;
 		int32_t hotspot_x = cur->state->buffer_hotspot.x;