PACKET: add PACKET_memdup and PACKET_strndup

Use each once in s3_srvr.c to show how they work.

Also fix a bug introduced in c3fc7eeab884b6876a1b4006163f190d325aa047
and made apparent by this change:
ssl3_get_next_proto wasn't updating next_proto_negotiated_len

Reviewed-by: Matt Caswell <matt@openssl.org>
3 files changed