Previously, we would call
g_datalist_id_set_data (&object->qdata, quark_closure_array, NULL);
which called destroy_closure_array() on the CArray.
At that point, it would iterate over the CArray, and invalidate all
closures. But note that this invokes external callbacks, which in turn
can destroy other closures, which can call object_remove_closure().
But now that closure can no longer be found and an assertion fails.
Instead of removing the entire CArray at once, remove each closure one
by one in a loop.
This problem is similar to issue 1002, except here it's about closure
watches instead of GWeakNotify.
Note that now we destroy closures one-by-one in a loop, and we iterate
the loop as long as we have closures. That makes a difference when a new
closure gets registered while we destroy them all. Previously, newly
registered closures would survive. It would be possible to implement the
previous behavior, but I think the new behavior is better. It is anyway
a very remote use case.
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