internet-apps: Fully reset V4TraceRoute output streams on final hop

The final-hop branch in Receive() reset the m_osRoute / m_routeIpv4 string streams with clear() only, which resets the state flags but does not empty the buffer, so stale route text could leak into a subsequent hop's output.

Pair str("") with clear(), as every other reset site in the file does.

Author: Gabrielcarvfer

src/internet-apps/model/v4traceroute.cc

@@ -333,7 +333,9 @@ V4TraceRoute::Receive(Ptr<Socket> socket)
                                                             << " " << m_osRoute.str() << "\n";
                             }
 
+                            m_osRoute.str("");
                             m_osRoute.clear();
+                            m_routeIpv4.str("");
                             m_routeIpv4.clear();
                         }
                         else