zmq::writer_t: Add missing test for swap

src/pipe.cpp

@@ -309,7 +309,6 @@ void zmq::writer_t::process_activate_writer (uint64_t msgs_read_)
         }
         if (!pipe->flush ())
             send_activate_reader (reader);
-    }
 
         //  There are no more messages in the swap. We can switch into
         //  standard in-memory mode.
@@ -329,6 +328,7 @@ void zmq::writer_t::process_activate_writer (uint64_t msgs_read_)
                 return;
             }
         }
+    }
 
     //  If the writer was non-active before, let's make it active
     //  (available for writing messages to).