Fixed an issue where attack values could underflow when a StatDown effect reduced a character's attack below zero.

Example:

• Current Physical Attack: 120
• Effect: <change stat="PHYSICAL_ATTACK" func="ADD" value="-150"/>
• Result: 120 - 150 = -30

Instead of displaying the correct value, the character stats window would overflow and incorrectly show +65390 attack for both physical and magical classes.