Bail out of traceroute if the initial TTL guess probe is not responded

to. Currently continuing from there leads to pathological behavior where
every hop up to 50 is tried, taking a very long time.

traceroute.cc

@@ -1144,6 +1144,9 @@ TraceGroup::retransmissions (vector < TraceProbe * >&retrans) {
             if (it->second->probeType () == PROBE_TTL) {
                 hopDistance = 1;
                 noDistProbe = true;
+                /* Give up on this host. We should be able to do a trace against
+                   an unresponsive target but for now it's too slow. */
+                setState(G_DEAD_TTL);
                 if (o.verbose)
                     log_write (LOG_STDOUT, "%s: no reply to our hop distance probe!\n", IPStr ());
             } else if (it->second->ttl > MAX_TTL) {