screen-cast: Fix segfault when drawing cursor

Add a sanity check if the cursor is on screen and cursor texture data
is available. This prevents a potential segfault when trying to access
non-existing texture data.

Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1446


(cherry picked from commit efb577ef)

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1606>