Remove superfluous check

|tls_process_finished| was checking that |peer_finish_md_len| was
non-negative. However neither |tls1_final_finish_mac| or
|ssl3_final_finish_mac| can ever return a negative value, so the check is
superfluous.

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>