Work around for LIBZMQ-496

The problem is that other threads might still be in mailbox::send() when
it is destroyed. So as a workaround, we just acquire the mutex in the
destructor. Therefore the running send will finish before the mailbox is
destroyed.

See also the fix for LIBZMQ-281 in zeromq2-x.
Signed-off-by: Mika Fischer <mika.fischer@zoopnet.de>

src/mailbox.cpp

@@ -33,6 +33,11 @@ zmq::mailbox_t::mailbox_t ()
 zmq::mailbox_t::~mailbox_t ()
 {
     //  TODO: Retrieve and deallocate commands inside the cpipe.
+
+    // Work around problem that other threads might still be in our
+    // send() method, by waiting on the mutex before disappearing.
+    sync.lock ();
+    sync.unlock ();
 }
 
 zmq::fd_t zmq::mailbox_t::get_fd ()