BUG/MINOR: acl: req_ssl_sni would randomly fail if a session ID is present

BUG/MINOR: acl: req_ssl_sni would randomly fail if a session ID is present

The wrong byte was checked for the session_id length in the payload. This
used to work when the session ID was absent because zero was found there,
but when a session ID is present, there is 1/256 chance that the inspected
data contains 0x20 (the actual session ID length), so it fails.

Thanks to Emmanuel Bézagu for reporting this bug.

This bug does not need backporting, it is 1.5 specific.