The function disassociate_js_gobject() is currently unconditionally
removing the weak reference of the wrapped GObject. This is because
we were previously assuming that there was always a toggle ref, and
so we would never reach this code path after the wrapped GObject was
destroyed.

Turns out, with commit 0cc23474 it is now a regular case to call
this function after the GObject is finalized. When that happens, it
will reach wrapped_gobj_dispose_notify() and the wrapped GObject will
be marked as finalized. disassociate_js_gobject(), however, will try
to weak unref it, without checking if the GObject was finalized already.

This commit fix that by simply checking if the wrapped GObject was
finalized before trying to weak unref it.