Clears SSL error state.

Updates tport_tls.c to clear the SSL error state after an error occurs.

OpenSSL puts errors into a queue that is kept in thread local storage.
In some cases, such as when SSL_ERROR_SSL is returned by SSL_get_errror(),
OpenSSL will queue multiple errors for a single event. When this occurs,
OpenSSL will report an error the next time I/O is performed if the queue
is not cleared first, which can result in TLS connections being torn down
prematurely.