Fix test of first of 255 CBC padding bytes.

Thanks to Peter Gijsels for pointing out that if a CBC record has 255
bytes of padding, the first was not being checked.

(This is an import of change 80842bdb from BoringSSL.)

Reviewed-by: Emilia Käsper <emilia@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1431)