Correct error handling for SSL_ERROR_SSL

[djones6]

Description

This PR fixes two things in the Linux (openssl) implementation:

• Inserts missing calls to ERR_clear_error prior to functions that may require SSL_get_error to be called
• Corrects the logic for obtaining the detailed error description for the SSL_ERROR_SSL case.

Motivation and Context

This resolves an intermittent failure exposed by a test added to Kitura-net (Kitura/Kitura-net#219), where the test can cause an SSL protocol error (SSL_ERROR_SSL).

When this occurs, an error is placed onto that thread's error queue, however the throwLastError function did not call ERR_get_error in this case, instead reporting "Could not determine error reason." and leaving the error on the queue.

The next SSL call with a non-zero return on that thread would then trigger a call to SSL_get_error, which would incorrectly pick up the error on the queue, incorrectly reporting another SSL_ERROR_SSL.

Part of the contract of SSL_get_error is that the error queue must have been cleared before a call to SSL_read, SSL_accept, etc. is made. To ensure we obey this contract, I have added ERR_clear_error before each such call.

How Has This Been Tested?

I have run the Kitura-net tests and they pass.

Since we do not have a test that deliberately or reliably causes an SSL protocol error, to test the output of the error reporting, I modified a test locally to remove the SSL delegate from the client-side socket and then send some erroneous data. This results in the following output:

[ERROR] [IncomingSocketHandler.swift:153 handleRead()] Read from socket (file descriptor 15) failed. Error = Error code: 336130315(0x1408F10B), ERROR: SSL_read, code: 336130315, reason: SSL3_GET_RECORD:wrong version number.

Checklist:

• I have submitted a CLA form
• If applicable, I have updated the documentation accordingly.
• If applicable, I have added tests to cover my changes.