Fix smtp starttls

[Flakebi]

In cases where the connection was fast, curl sometimes failed to open a connection.
This fixes a regression of c2d9736.

The regression triggered in these steps:

1. Create an smtp connection

2. Use STARTTLS

3. Receive the response

4. We are inside the loop in smtp_statemachine, calling smtp_state_starttls_resp

5. In the good flow, we exit the loop, re-enter smtp_statemachine and run smtp_perform_upgrade_tls at the start of the function.

   In the bad flow, we stay in the while loop, calling Curl_pp_readresp, which reads part of the TLS handshake and things go wrong.

The reason is that Curl_pp_moredata changed behavior and always returns true, so we stay in the loop in smtp_statemachine. With a slow connection Curl_pp_readresp cannot read new data and returns CURL_AGAIN, so we leave the loop and re-enter smtp_statemachine.

With a fast connection, Curl_pp_readresp reads new data from the tcp connection, which is part of the TLS handshake.

The fix is in Curl_pp_moredata, which needs to take the final line into account and return false if only the final line is stored.

A test would be nice, but I’m not accustomed to the testing system, so I there’s no test in this change.

[bagder]

Thanks!