Ensure SSL_set_session clears the old session from cache if it is bad

SSL_clear() and SSL_free() will remove a session from the cache if it is
considered "bad". However SSL_set_session() does not do this for the session
it is replacing.

SSL_clear() clears an SSL object ready for reuse. It does not clear the
session though. This means that:

  SSL_clear(s)
  SSL_set_session(s, sess);

and
  SSL_set_session(s, sess);
  SSL_clear(s);

do not do the same thing, although logically you would expect that they
would.

The failure of SSL_set_session() to remove bad sessions from the cache
should be considered a bug, so this commit fixes it.

RT#597

Reviewed-by: Rich Salz <rsalz@openssl.org>