Prevent Gtk-CRITICAL when unrealize-ing a GtkPaned.

This commit fixes a bug (issue #2066 (closed)) caused by gdk_window_set_cursor being called with a NULL pointer as its first argument during the unrealize-ing of a GtkPaned, resulting in a Gtk-CRITICAL. The change ensures that, even if the function gtk_paned_state_flags_changed is called on a GtkPaned while it is part way through being unrealized (as can happen, for example, if a descendant of the GtkPaned calls gtk_clipboard_store when it is unrealize-d, see the issue report for a test case), the function gdk_window_set_cursor is not called on the GtkPaned's priv->handle member if this pointer is NULL.

Closes #2066 (closed) Fixes #2066 (closed) Closes: #2066 (closed)