Protect against GCing of last_mouse_window

Protect against GCing of last_mouse_window

* src/window.c (last_mouse_window): New global var.  All static
instances removed, and all their uses replaced with this global
var.  This fixes a very unlikely bug where last_mouse_window was
GC’ed and a new window created in its place.  It also fixes several
places that assumed NIL_IS_ZERO without static_asserting it.
(init_window_once): Initialize the new var.

(cherry picked from commit 2e8ef0910412aef8f9f1beba7c942473ad8602bb)