Don't allow matching probes that haven't been sent.

This is a reversion of r26232 and r26230, which were themselves
reversions of r26201 and r26202 respectively, in
https://svn.nmap.org/nmap-exp/luis/nmap-os6@26232.

This code has gone back and forth a few times. Looking at it again, I
still think this way is more correct. At any rate, the other way
demonstrably leads to rare assertion failures (which are protecting
against a nonsensical subtraction with an all-zero timeval).

CHANGELOG

@@ -27,6 +27,10 @@ o [NSE] Added a stun library and the scripts stun-version and stun-info, which
 o [NSE] Added the script duplicates which attempts to determine duplicate
   hosts by analyzing information collected by other scripts. [Patrik Karlsson]
 
+o Changed the way timeout calculations are made in the IPv6 OS engine.
+  In rare cases a certain interleaving of probes and responses would
+  result in an assertion failure.
+
 Nmap 5.61TEST5 [2012-03-09]
 
 o Integrated all of your IPv4 OS fingerprint submissions since June

FPEngine.cc

@@ -2000,7 +2000,6 @@ int FPHost6::schedule() {
               this->fp_probes[i].getRetransmissions());
           }
           this->fp_probes[i].incrementRetransmissions();
-          this->fp_probes[i].resetTimeSent();
           this->netctl->scheduleProbe(&(this->fp_probes[i]), 0);
           break;
         }
@@ -2128,7 +2127,6 @@ int FPHost6::schedule() {
       int whentostart = get_random_u16()%100;
       for (size_t l = 0; l < this->timed_probes; l++) {
         this->fp_probes[l].incrementRetransmissions();
-        this->fp_probes[l].resetTimeSent();
         this->netctl->scheduleProbe(&(this->fp_probes[l]), whentostart + l*100);
       }
       if (o.debugging > 3 && this->timed_probes > 0)
@@ -2516,6 +2514,11 @@ void FPProbe::reset() {
  * PacketParser::is_response(). Check there for a list of matched packets and
  * some usage examples.*/
 bool FPProbe::isResponse(PacketElement *rcvd) {
+  /* If we don't have a record of even sending this probe, no packet can be a
+     response. */
+  if (this->pkt_time.tv_sec == 0 && this->pkt_time.tv_usec == 0)
+    return false;
+
   return PacketParser::is_response(this->pkt, rcvd);
 }