compat: nn_reallocmsg is incorrect

[Snarpix]

nng/src/compat/nanomsg/nn.c

Lines 366 to 379 in a54820f

	// This counts on message bodies being aligned sensibly.
	msg = *(nng_msg **) (((char *) ptr) - sizeof(msg));
	// We need to realloc the requested len, plus size for our header.
	if ((rv = nng_msg_realloc(msg, len + sizeof(msg))) != 0) {
	// We don't free the old message. Code is free to cope
	// as it sees fit.
	nn_seterror(rv);
	return (NULL);
	}
	// Stash the msg header pointer
	*(nng_msg **) (nng_msg_body(msg)) = msg;
	nng_msg_trim(msg, sizeof(msg));
	return (nng_msg_body(msg));

In this code nng_msg_realloc copies message without trimmed nng_msg* in the beginning.
Then we overwrite first 8 message bytes with nng_msg*, and trim them, so on realloc we are actually removing first 8 bytes of message.

[gdamore]

Upon reading, I agree. There is no test for this code, and I don't use it. I should fix this, and a test case for it.