Fix V803 Decreased performance. In case 'it' is iterator it's more effective to…

Fix V803 Decreased performance. In case 'it' is iterator it's more effective to use prefix form of increment.
    Replace iterator++ with ++iterator. stream_engine.cpp
    http://www.viva64.com/en/d/0165/print/

src/stream_engine.cpp

@@ -781,7 +781,7 @@ void zmq::stream_engine_t::mechanism_ready ()
     it = zap_properties.begin ();
     while (it != zap_properties.end ()) {
         properties.insert (properties_t::value_type (it->first, it->second));
-        it++;
+        ++it;
     }
 
     //  Add ZMTP properties.
@@ -789,7 +789,7 @@ void zmq::stream_engine_t::mechanism_ready ()
     it = zmtp_properties.begin ();
     while (it != zmtp_properties.end ()) {
         properties.insert (properties_t::value_type (it->first, it->second));
-        it++;
+        ++it;
     }
 
     zmq_assert (metadata == NULL);