fix memset overflow in oldmalloc race fix overhaul

commit 3e16313f8f 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.

src/malloc/oldmalloc/malloc.c

@@ -409,7 +409,7 @@ copy_realloc:
 	new = malloc(n-OVERHEAD);
 	if (!new) return 0;
 copy_free_ret:
-	memcpy(new, p, n0-OVERHEAD);
+	memcpy(new, p, (n<n0 ? n : n0) - OVERHEAD);
 	free(CHUNK_TO_MEM(self));
 	return new;
 }