Ensure that handshake hash is non-NULL in tls1_transcript_hash_value().

Ensure that handshake hash is non-NULL in tls1_transcript_hash_value().

There are several paths where a subtle bug could result in
tls1_transcript_hash_value() being called with a NULL handshake hash - add
an explicit check for this case. As noted by tb@, due to the wonders of
the libcrypto EVP APIs, combined with integer promotion, we already have
a NULL check - this one is just more obvious.

ok tb@