Avoid clobbering the error code when sending an alert

Avoid clobbering the error code when sending an alert

In order to fail gracefully on encountering a self-signed cert, curl looks
at the top-most error on the stack and needs specific SSL_R_ error codes.
This mechanism was broken when the tls13_alert_sent_cb() was added after
people complained about unhelpful unknown errors. Fix this by only setting
the error code from a fatal alert if no error has been set previously.

Issue reported by Christopher Reid

ok jsing