PROTON-2785: Fix pn_data_clear not clearing error

[PatrickTaibel]

We've also hit the bug of the Go library that is described in PROTON-2785. This bug happens when one large message is sent and another message with arbitrary size gets marshaled.

Reason is that the large message does multiple calls with a too small message size to pn_data_encode which leads to the error PN_OVERFLOW to be set in pn_encoder_encode on the data field until a large enough buffer is provided. As the pn_message_t is reused the error hits the second message encoding.

Relevant Go code:

qpid-proton/go/pkg/amqp/message.go

Lines 378 to 393 in 9fdc19c

	func (mc *MessageCodec) Encode(m Message, buffer []byte) ([]byte, error) {
	pn := mc.pnMessage()
	m.(*message).put(pn)
	encode := func(buf []byte) ([]byte, error) {
	len := cLen(buf)
	result := C.pn_message_encode(pn, cPtr(buf), &len)
	switch {
	case result == C.PN_OVERFLOW:
	return buf, overflow
	case result < 0:
	return buf, fmt.Errorf("cannot encode message: %s", PnErrorCode(result))
	default:
	return buf[:len], nil
	}
	}
	return encodeGrow(buffer, encode)

Relevant C code:

qpid-proton/c/src/core/encoder.c

Lines 395 to 409 in 9fdc19c

	ssize_t pn_encoder_encode(pn_encoder_t *encoder, pn_data_t *src, char *dst, size_t size)
	{
	encoder->output = dst;
	encoder->position = 0;
	encoder->size = size;
	int err = pni_data_traverse(src, pni_encoder_enter, pni_encoder_exit, encoder);
	if (err) return err;
	size_t encoded = encoder->position;
	if (encoded > size) {
	pn_error_format(pn_data_error(src), PN_OVERFLOW, "not enough space to encode");
	return PN_OVERFLOW;
	}
	return (ssize_t)encoded;
	}

This PR adds clearing the error field when pn_data_clear is called. According to the docs "A cleared pn_data_t object is equivalent to a newly constructed one.", so that would match this fix.

Other possible way would be to not set those PN_OVERFLOW errors on pn_data_t at all. I don't have enough overview of the internals to know if this makes sense or not. Especially, as there are a few other errors that are set on pn_data_t too which might cause similar issues.