It feels ugly to leave the buffer not sized right.
We call g_object_weak_release_all() during g_object_real_dispose() and
right before finalize. In most cases, we expect that the loop iterates
until there are no weak notifications left (in which case the entire
WeakRefStack is freed). In that case, there is no need to shrink the
buffer, because it's going to be released soon anyway.
Note that no new weak references can be registered after finalize (as
the ref count already dropped to zero). However, new weak referenes can
be registered during dispose (either during the last g_object_unref() or
during g_object_run_dispose()).
In that case, I feel it is nice to bring the buffer size right again. We
don't know how long the object will continue to live afterwards, so
let's trim the extra allocation.
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