Core/Pets: Clean up logic/codedesign slightly for XP/levelup-handling, and also fix a possible overflow in the pet level-tests

(Thanks to Vladimir for the discussion regarding the possible overflow!)

--HG--
branch : trunk

src/server/game/Entities/Unit/Unit.cpp

@@ -14852,7 +14852,7 @@ Pet* Unit::CreateTamedPetFrom(Creature* creatureTarget,uint32 spell_id)
         return NULL;
     }
 
-    uint8 level = (creatureTarget->getLevel() < (getLevel() - 5)) ? (getLevel() - 5) : creatureTarget->getLevel();
+    uint8 level = creatureTarget->getLevel() + 5 < getLevel() ? (getLevel() - 5) : creatureTarget->getLevel();
 
     InitTamedPet(pet, level, spell_id);