Fix DTLS handshake fragment retries

Fix DTLS handshake fragment retries

If using DTLS and NBIO then if a second or subsequent handshake message
fragment hits a retry, then the retry attempt uses the wrong fragment
offset value. This commit restores the fragment offset from the last
attempt.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(cherry picked from commit 2ad226e88bee97847496e542d63c67997d5beda6)