wayland: Don't crash on cursor size 0

The cursor-theme-size setting is documented as
'0 means the default size'. Make it so by using
size 24 if we see a 0. Its better than crashing.

Fixes: #5700

gdk/wayland/gdkdisplay-wayland.c

@@ -1082,6 +1082,9 @@ gdk_wayland_display_set_cursor_theme (GdkDisplay *display,
   g_assert (display_wayland);
   g_assert (display_wayland->shm);
 
+  if (size == 0)
+    size = 24;
+
   if (g_strcmp0 (name, display_wayland->cursor_theme_name) == 0 &&
       display_wayland->cursor_theme_size == size)
     return;