Core/Unit: fix logic error in DamageInfo::ModifyDamage.

Previous code did not protect against m_damage underflow, rather only allow up to duplicate damage.
It now should work as intended.

Closes #18154

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

@@ -161,7 +161,8 @@ DamageInfo::DamageInfo(SpellNonMeleeDamage const& spellNonMeleeDamage, DamageEff
 
 void DamageInfo::ModifyDamage(int32 amount)
 {
-    amount = std::min(amount, int32(GetDamage()));
+    if (amount < 0)
+        amount = std::max(amount, static_cast<int32>(-GetDamage()));
     m_damage += amount;
 }