Avoid transcript initialisation when sending a TLS HelloRequest.

Avoid transcript initialisation when sending a TLS HelloRequest.

When server side renegotiation is triggered, the TLSv1.2 state machine
sends a HelloRequest before going to ST_SW_FLUSH and ST_OK. In this case
we do not need the transcript and currently hit the sanity check in ST_OK
that ensures the transcript has been freed, breaking server initiated
renegotiation. We do however need the transcript in the DTLS case.

ok tb@