xdg-shell: Add invalid_parent error to xdg_popup

Send an invalid_parent error when the client tries to create a popup
with a paren that is neither a xdg_surface nor a xdg_popup.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
diff --git a/desktop-shell/shell.c b/desktop-shell/shell.c
index b326d68..bec49e6 100644
--- a/desktop-shell/shell.c
+++ b/desktop-shell/shell.c
@@ -4092,12 +4092,21 @@
 		 uint32_t serial,
 		 int32_t x, int32_t y)
 {
-	struct shell_surface *shsurf;
+	struct shell_surface *shsurf, *parent_shsurf;
 
 	/* Verify that we are creating the top most popup when mapping,
 	 * as its not until then we know whether it was mapped as most
 	 * top level or not. */
 
+	parent_shsurf = get_shell_surface(parent);
+	if (!shell_surface_is_xdg_popup(parent_shsurf) &&
+	    !shell_surface_is_xdg_surface(parent_shsurf)) {
+		wl_resource_post_error(owner->resource,
+				       XDG_POPUP_ERROR_INVALID_PARENT,
+				       "xdg_popup parent was invalid");
+		return NULL;
+	}
+
 	shsurf = create_common_surface(owner, shell, surface, client);
 	if (!shsurf)
 		return NULL;