Core/Auras: fix a typo in PowerDrain auras periodic tick handler.

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

@@ -12309,6 +12309,7 @@ int32 Unit::GetHealthGain(int32 dVal)
     return gain;
 }
 
+// returns negative amount on power reduction
 int32 Unit::ModifyPower(Powers power, int32 dVal)
 {
     int32 gain = 0;

src/server/game/Spells/Auras/SpellAuraEffects.cpp

@@ -6435,7 +6435,7 @@ void AuraEffect::HandlePeriodicManaLeechAuraTick(Unit* target, Unit* caster) con
     if (powerType == POWER_MANA)
         drainAmount -= target->GetSpellCritDamageReduction(drainAmount);
 
-    int32 drainedAmount = target->ModifyPower(powerType, -drainAmount);
+    int32 drainedAmount = -target->ModifyPower(powerType, -drainAmount);
 
     float gainMultiplier = SpellMgr::CalculateSpellEffectValueMultiplier(GetSpellProto(), GetEffIndex(), caster);