Problem: unfinished message can be leaked by client pipe

When a pipe processes a delimiter and is already not in active state but still
has an unfinished message, the message is leaked.

Solution: issue a rollback before losing the reference to the pipe.
(cherry picked from commit 6815138501b9f2a69e807bc3527d93583e633233)

Conflicts:
	src/pipe.cpp

src/pipe.cpp

@@ -451,6 +451,7 @@ void zmq::pipe_t::process_delimiter ()
     if (state == active)
         state = delimiter_received;
     else {
+        rollback ();
         outpipe = NULL;
         send_pipe_term_ack (peer);
         state = term_ack_sent;