Entities/Unit: Add a missing underflow check to prevent really really low damage from underflowing when paired with flat damage modifiers.

Port From (https://github.com/TrinityCore/TrinityCore/commit/970a89a8c3e615244b5968e04d888100fba2cff0)

Source/Game/Entities/Unit/Unit.cs

@@ -3676,6 +3676,9 @@ namespace Game.Entities
             else
                 TakenFlatBenefit += GetTotalAuraModifier(AuraType.ModRangedDamageTaken);
 
+            if ((TakenFlatBenefit < 0) && (pdamage < -TakenFlatBenefit))
+                return 0;
+
             // Taken total percent damage auras
             float TakenTotalMod = 1.0f;