workspace: Check if index is valid before using it

workspace: Check if index is valid before using it

If the window wasn't in _windowSlots, the index is -1, so the last
element of the array is removed, leading to
https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3547

I logged the stack trace from removeWindow() and it seems that when you
move the window from the second monitor to another workspace like shown
in https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3547,
removeWindow() is called twice for it, presumably once for the second
monitor workspace, and then once for the first monitor workspace for
some reason. It is during that call that _windowSlots doesn't contain
the window, so instead the last element (index -1) is removed, leading
to the animation bug.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1727>