When zmq::req_t::xrecv detects that a response has no request ID
label, or the ID is the wrong size, it would return an EAGAIN, but
would not discard the remainder of the message.  This could allow the
remainder of the message to incorrectly "leak" into a future response,
if it is crafted to look like a reply with a valid response ID.
Discard all remaining message blocks, if the ID is invalid in any way.