fix memset overflow in oldmalloc race fix overhaul

fix memset overflow in oldmalloc race fix overhaul

commit 3e16313f8fe2ed143ae0267fd79d63014c24779f introduced this bug by
making the copy case reachable with n (new size) smaller than n0
(original size). this was left as the only way of shrinking an
allocation because it reduces fragmentation if a free chunk of the
appropriate size is available. when that's not the case, another
approach may be better, but any such improvement would be independent
of fixing this bug.