crypto: remove unused access of tlsext_hostname

The return value of loadSession is ultimately ignored, so don't fill it
in. This inches Node closer to 1.1.0 compatibility and is less code.

Also remove a comment which appears to have long since become invalid.
It dates to 048e0e77 when the SNI value
was actually extracted from the session.

This also fixes a segfault should d2i_SSL_SESSION fail to parse the
input and return NULL. Add a test for this case based on
test-tls-session-cache.js.

PR-URL: https://github.com/nodejs/node/pull/10882


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>