Core/Threat: Prevent bosses (and other stuff using DoZoneInCombat) from switching off of the person pulling if they are pulled without generating threat.

(cherry picked from commit b5b7ce44cfeb98bf47eb6c9e0c926994ad53ede8)

src/server/game/AI/CreatureAI.cpp

@@ -134,7 +134,10 @@ void CreatureAI::MoveInLineOfSight(Unit* who)
         return;
 
     if (me->HasReactState(REACT_AGGRESSIVE) && me->CanStartAttack(who, false))
+    {
+        me->AddThreat(who, 0.0f); // ensure our initial target is the first thing added to threat list so we don't randomly switch off if DoZoneInCombat is called during the EnterCombat hook
         AttackStart(who);
+    }
     //else if (who->GetVictim() && me->IsFriendlyTo(who)
     //    && me->IsWithinDistInMap(who, sWorld->getIntConfig(CONFIG_CREATURE_FAMILY_ASSISTANCE_RADIUS))
     //    && me->CanStartAttack(who->GetVictim(), true)) /// @todo if we use true, it will not attack it when it arrives

src/server/game/Combat/ThreatManager.cpp

@@ -336,7 +336,7 @@ HostileReference* ThreatContainer::selectNextVictim(Creature* attacker, HostileR
             {
                 // current victim is a second choice target, so don't compare threat with it below
                 if (currentRef == currentVictim)
-                    currentVictim = NULL;
+                    currentVictim = nullptr;
                 ++iter;
                 continue;
             }
@@ -438,12 +438,15 @@ void ThreatManager::_addThreat(Unit* victim, float threat)
 
     if (!ref) // there was no ref => create a new one
     {
+        bool isFirst = iThreatContainer.empty();
                                                             // threat has to be 0 here
         HostileReference* hostileRef = new HostileReference(victim, this, 0);
         iThreatContainer.addReference(hostileRef);
         hostileRef->addThreat(threat); // now we add the real threat
         if (victim->GetTypeId() == TYPEID_PLAYER && victim->ToPlayer()->IsGameMaster())
             hostileRef->setOnlineOfflineState(false); // GM is always offline
+        else if (isFirst)
+            setCurrentVictim(hostileRef);
     }
 }
 

src/server/game/Combat/ThreatManager.h

@@ -169,7 +169,7 @@ class TC_GAME_API ThreatContainer
 
         HostileReference* getMostHated() const
         {
-            return iThreatList.empty() ? NULL : iThreatList.front();
+            return iThreatList.empty() ? nullptr : iThreatList.front();
         }
 
         HostileReference* getReferenceByTarget(Unit* victim) const;