Only call ssl3_init_finished_mac once for DTLS

In DTLS if an IO retry occurs during writing of a fragmented ClientHello
then we can end up reseting the finish mac variables on the retry, which
causes a handshake failure. We should only reset on the first attempt not
on retries.

Thanks to BoringSSL for reporting this issue.

RT#4119

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