Revert "problem:Socket try to access closed mutex"

src/socket_base.cpp

@@ -1514,12 +1514,14 @@ void zmq::socket_base_t::in_event ()
     //  of the reaper thread. Process any commands from other threads/sockets
     //  that may be available at the moment. Ultimately, the socket will
     //  be destroyed.
+    ENTER_MUTEX ();
 
     //  If the socket is thread safe we need to unsignal the reaper signaler
     if (thread_safe)
         reaper_signaler->recv();
 
     process_commands (0, false);
+    EXIT_MUTEX();
     check_destroy();
 }