Prevent calling SSL_set_session in the middle of handshake

[maskit]

ATS crashes if origin session cache is used with BoringSSL, because BoringSSL doesn't allow you to call SSL_set_session after starting handshake.

The crash is not 100% reproducible and I was only able to reproduce it on 9.2.x, but the change should make sense on master as well.

https://www.openssl.org/docs/man1.1.1/man3/SSL_set_session.html

If there is already a session set inside ssl (because it was set with SSL_set_session() before or because the same ssl was already used for a connection), SSL_SESSION_free() will be called for that session. If that old session is still open, it is considered bad and will be removed from the session cache (if used). A session is considered open, if SSL_shutdown(3) was not called for the connection (or at least SSL_set_shutdown(3) was used to set the SSL_SENT_SHUTDOWN state).

https://commondatastorage.googleapis.com/chromium-boringssl-docs/ssl.h.html#SSL_set_session

It is an error to call this function after the handshake has begun.

The change is very small if you hide whitespace (indent) changes.

[zwoop]

Cherry-picked to v9.2.x