TLS: clear error queue before doing IO or calling SSL_get_error()

TLS: clear error queue before doing IO or calling SSL_get_error()

Ensure that TLS error is empty before calling SSL_get_error() or doing
SSL I/O so that the result will not get affected by prior error
statuses.

In particular, the improper error handling led to intermittent unit
test failure and, thus, could be responsible for some of the system
test failures and other intermittent TLS-related issues.

See here for more details:

https://www.openssl.org/docs/man3.0/man3/SSL_get_error.html

In particular, it mentions the following:

> The current thread's error queue must be empty before the TLS/SSL
> I/O operation is attempted, or SSL_get_error() will not work
> reliably.

As we use the result of SSL_get_error() to decide on I/O operations,
we need to ensure that it works reliably by cleaning the error queue.

TLS DNS: empty error queue before attempting I/O