crypto: clear openssl error stack when handled

Clear OpenSSL's error stack on return from Connection::HandleSSLError().
This stops stale errors from popping up later in the lifecycle of the
SSL connection where they would cause spurious failures.

This commit causes a 1-2% performance regression on `make bench-tls`.
We'll address that in follow-up commits if possible but let's ensure
correctness first.

Backport of c6e2db2