object: Only weak unref if GObject is still alive

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.