Check EVP errors for handshake digests.

Partial mitigation of PR#3200
sha512.c: fullfull implicit API contract in SHA512_Transform.

SHA512_Transform was initially added rather as tribute to tradition
than for practucal reasons. But use was recently found in ssl/s3_cbc.c
and it turned to be problematic on platforms that don't tolerate
misasligned references to memory and lack assembly subroutine.
(cherry picked from commit cdd1acd788020d2c525331da1712ada778f1373c)