buffer_list_aggregate_separator(): don't exceed max_len

buffer_list_aggregate_separator(): don't exceed max_len

buffer_list_aggregate_separator() would merge buffer_list entries until it
had exceeded the provided max_len, instead of stopping *before* exceeding
the max value.

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <1514541191-19471-1-git-send-email-steffan.karger@fox-it.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16104.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit fb6138dd32cf01922d7ef670d502148596511268)