Fixes crash with touchscreens when attempting to drag windows

https://gitlab.gnome.org/GNOME/mutter/-/work_items/4887

--- a/src/wayland/meta-wayland-touch.c
+++ b/src/wayland/meta-wayland-touch.c
@@ -659,3 +659,3 @@
-  if (touch_info->focus_surface->surface == surface)
+  if (touch_info->focus_surface && touch_info->focus_surface->surface == surface)