Core/Pet: Add assert

Add additional assert to check if it's possible for a Pet not to have an active AI when the Player owner kills an enemy.
Ref #23036

(cherry picked from commit 43dec9e4d0b6f7da3c2051624742e9fb8a9bcb31)

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

@@ -10483,7 +10483,7 @@ void Unit::SetMeleeAnimKitId(uint16 animKitId)
     {
         Pet* pet = player->GetPet();
         if (pet && pet->IsAlive() && pet->isControlled())
-            pet->AI()->KilledUnit(victim);
+            ASSERT_NOTNULL(pet->AI())->KilledUnit(victim);
     }
 
     // 10% durability loss on death