Core/Pets: Fixed crash when summoning hunter pets

Port From (https://github.com/TrinityCore/TrinityCore/commit/074c5da6e43796a88ad34e8724362433d5109f88)

Source/Game/Entities/Creature/Creature.cs

@@ -220,7 +220,7 @@ namespace Game.Entities
 
             m_creatureInfo = creatureInfo;
             SetEntry(entry);
-            m_creatureDifficulty = creatureInfo.GetDifficulty(GetMap().GetDifficultyID());
+            m_creatureDifficulty = creatureInfo.GetDifficulty(!IsPet() ? GetMap().GetDifficultyID() : Difficulty.None);
 
             // equal to player Race field, but creature does not have race
             SetRace(0);

Source/Game/Entities/Pet.cs

@@ -162,7 +162,11 @@ namespace Game.Entities
             if (petInfo.Type == PetType.Hunter)
             {
                 CreatureTemplate creatureInfo = Global.ObjectMgr.GetCreatureTemplate(petInfo.CreatureId);
-                if (creatureInfo == null || !creatureInfo.IsTameable(owner.CanTameExoticPets(), GetCreatureDifficulty()))
+                if (creatureInfo == null)
+                    return false;
+
+                CreatureDifficulty creatureDifficulty = creatureInfo.GetDifficulty(Difficulty.None);
+                if (creatureDifficulty == null || !creatureInfo.IsTameable(owner.CanTameExoticPets(), creatureDifficulty))
                     return false;
             }