Scripts/Achievements: Fixed a crash in Heigan's achievement script & more possible crashes & small improvements to handling datas for achievements

src/server/scripts/Northrend/DraktharonKeep/boss_dred.cpp

@@ -283,6 +283,9 @@ class achievement_king_dred : public AchievementCriteriaScript
 
         bool OnCheck(Player* /*player*/, Unit* target)
         {
+            if (!target)
+                return false;
+
             if (Creature* Dred = target->ToCreature())
                 if (Dred->AI()->GetData(DATA_KING_DRED) >= 6)
                     return true;

src/server/scripts/Northrend/DraktharonKeep/boss_novos.cpp

@@ -352,6 +352,9 @@ class achievement_oh_novos : public AchievementCriteriaScript
 
         bool OnCheck(Player* /*player*/, Unit* target)
         {
+            if (!target)
+                return false;
+
             if (Creature* Novos = target->ToCreature())
                 if (Novos->AI()->GetData(DATA_OH_NOVOS))
                     return true;

src/server/scripts/Northrend/DraktharonKeep/boss_trollgore.cpp

@@ -204,6 +204,9 @@ class achievement_consumption_junction : public AchievementCriteriaScript
 
         bool OnCheck(Player* /*player*/, Unit* target)
         {
+            if (!target)
+                return false;
+
             if (Creature* Trollgore = target->ToCreature())
                 if (Trollgore->AI()->GetData(DATA_CONSUMPTION_JUNCTION))
                     return true;