dispose() can resurrect an object and/or register a weak-ref. After
returning from dispose(), we must check again. And we must do so in a
race-free manner, where we check that we have no more weak-locations
and the ref-count is one.
In fact, if _object_unref_clear_weak_locations() determines that the
ref-count is 1, it must also decrement the ref-count to zero while
holding the weak_locations_lock. This prevents g_weak_ref_set() to
still register a weak-pointer after the reference count dropped to zero.
Also add an assertion to g_weak_ref_set(), that the object is still
alive. The assertion is useful to finding bugs, but the change really
makes it impossible that a wrongly used g_weak_ref_set() can still
resurrect the object after finalization starts.
The final
g_datalist_id_set_data (&object->qdata, quark_weak_locations, NULL);
during finalization is no longer necessary and dropped.
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