LibIPC: Join send thread instead of detaching in ~ConnectionBase()

By doing this we ensure that sending thread won't try to write into a
closed socket.

Libraries/LibIPC/Connection.cpp

@@ -69,7 +69,7 @@ ConnectionBase::~ConnectionBase()
         m_send_queue->running = false;
         m_send_queue->condition.signal();
     }
-    m_send_thread->detach();
+    (void)m_send_thread->join();
 }
 
 bool ConnectionBase::is_open() const