Fix small OOB reads.

In ssl3_get_client_certificate, ssl3_get_server_certificate and
ssl3_get_certificate_request check we have enough room
before reading a length.

Thanks to Shi Lei (Gear Team, Qihoo 360 Inc.) for reporting these bugs.

CVE-2016-6306

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>