Authentication errors are sometimes not read if backend TLS is enabled

If the server sends an authentication error and then proceeds to close the connection immediately, OpenSSL seems to return zero for both SSL_read() and SSL_get_error(). Previously in 21.06 this was treated as a success case if data was read from the socket but in 23.08 it's always treated as an error. The effect of this is that the reported error is Read from backend failed instead of an authentication error.