pack-objects: clamp negative window size to 0

A negative window size makes no sense, and the code in find_deltas() is not prepared to handle it. If you pass "-1", for example, we end up generate a 0-length array of "struct unpacked", but our loop assumes it has at least one entry in it (and we end up reading garbage memory). We could complain to the user about this, but it's more forgiving to just clamp it to 0, which means "do not find any deltas at all". The 0-case is already tested earlier in the script, so we'll make sure this does the same thing. Reported-by: Yiyuan guo <yguoaz@gmail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>