asyncnet, net: clear openssl error queue before performing I/O

Per SSL_get_error(3):

  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.

There has been records of not clearing the error queue causing weird SSL
errors when there shouldn't be any, see:

https://github.com/openssl/openssl/issues/11889

lib/pure/asyncnet.nim

@@ -718,6 +718,7 @@ proc close*(socket: AsyncSocket) =
         # established, see:
         # https://github.com/openssl/openssl/issues/710#issuecomment-253897666
         if SSL_in_init(socket.sslHandle) == 0:
+          ErrClearError()
           SSL_shutdown(socket.sslHandle)
         else:
           0