Initialize the session_id

ssl_session_hash() always looks at the first 4 bytes, regardless of the length.
A client can send a session id that's shorter, and the callback could also
generate one that's shorter.  So we make sure that the rest of the buffer is
initliazed to 0 so that we always calculate the same hash.

Found by tis-interpreter, also previously reported as RT #2871

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

MR: #2911