transport-helper.c: fix check for (size_t < 0)

'bytes' is of type size_t which is unsigned thus can't be negative.  But
the assigned write() returns ssize_t, and -1 on error.

For testing < 0, 'bytes' needs to be of a signed type.

Signed-off-by: Nicolas Kaiser <nikai@nikai.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>