UDP engine out_event aborts when sendto fails

[simias]

Issue description

The current implementation of udp_engine triggers an assert when sendto fails. This makes it hard to recover from the error.

libzmq/src/udp_engine.cpp

Lines 392 to 403 in 6092431

	#ifdef ZMQ_HAVE_WINDOWS
	rc = sendto (fd, (const char *) out_buffer, (int) size, 0, out_address,
	(int) out_addrlen);
	wsa_assert (rc != SOCKET_ERROR);
	#elif defined ZMQ_HAVE_VXWORKS
	rc = sendto (fd, (caddr_t) out_buffer, size, 0,
	(sockaddr *) out_address, (int) out_addrlen);
	errno_assert (rc != -1);
	#else
	rc = sendto (fd, out_buffer, size, 0, out_address, out_addrlen);
	errno_assert (rc != -1);
	#endif

I'm not sure what's the proper way to address this issue given that out_event returns void.

Environment

• libzmq version (commit hash if unreleased): current master (6092431)
• OS: Any

Minimal test code / Steps to reproduce the issue

I'm having the issue when binding a multicast RADIO to an interface that doesn't have the link yet. It causes my python app to abort brutally.

What's the actual result? (include assertion message & call stack if applicable)

Abort

What's the expected result?

Return an error to let the calling code handle the situation

[bluca]

I think this is the same as #2862 and yes it should get fixed