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

tests/stdlib/thttpclient_ssl.nim

@@ -61,6 +61,7 @@ when not defined(windows):
     var ssl: SslPtr = SSL_new(ctx.context)
     discard SSL_set_fd(ssl, client.getFd())
     log "server: accepting connection"
+    ErrClearError()
     if SSL_accept(ssl) <= 0:
       ERR_print_errors_fp(stderr)
     else: