Fix socket_t::recv() hang scenario where initial call to process_commands() eats signal

Added block boolean var to second process_commands() invocation for blocking sockets
instead of always using true.  This prevents the process_commands() call from hanging
when a message is received with an empty queue after the call to xrecv() but
prior to the initial call to process_commands() invoked when ++ticks == inbound_poll_rate.
Signed-off-by: Marc Rossi <mrossi19@gmail.com>

AUTHORS

@@ -20,6 +20,7 @@ Ivo Danihelka <ivo@danihelka.net>
 Joe Thornber <joe.thornber@gmail.com>
 Jon Dyte <jon@totient.co.uk>
 Kamil Shakirov <kamils80@gmail.com>
+Marc Rossi <mrossi19@gmail.com>
 Martin Hurton <hurtonm@gmail.com>
 Martin Lucina <mato@kotelna.sk>
 Martin Sustrik <sustrik@250bpm.com>

src/socket_base.cpp

@@ -437,15 +437,17 @@ int zmq::socket_base_t::recv (::zmq_msg_t *msg_, int flags_)
 
     //  In blocking scenario, commands are processed over and over again until
     //  we are able to fetch a message.
+    bool block = (ticks != 0);
     while (rc != 0) {
         if (errno != EAGAIN)
             return -1;
-        if (unlikely (!app_thread->process_commands (true, false))) {
+        if (unlikely (!app_thread->process_commands (block, false))) {
             errno = ETERM;
             return -1;
         }
         rc = xrecv (msg_, flags_);
         ticks = 0;
+        block = true;
     }
 
     rcvmore = msg_->flags & ZMQ_MSG_MORE;