Fix a ssl session leak due to OOM in lh_SSL_SESSION_insert

- s == NULL can mean c is a new session *or* lh_insert was
  unable to create a hash entry.
- use lh_SSL_SESSION_retrieve to check for this error condition.
- If it happens simply remove the extra reference again.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2138)