Fix item amount underflow on item deflect

This fixes an underflow issue when a player uses an item within the same time they get hurt by that same item.

Bug in question:

ringracers0140

What causes the underflow? The following happens in the same tic:

• Within K_MoveKartPlayer's itemtype switch (L12767 onwards), player uses their item, calling K_ThrowKartItem.
• Item spawns, which gets deflected immediately through K_BubbleShieldReflect transfering ownership (making threshold or leniency frames not apply anymore).
• This hits the player back, which runs K_DropHnextList. This sets the player's item amount to zero and their item type to none.
• We didn't decrease the item amount yet, so we set item amount - 1.
• This underflows into having 255 of an empty item.

The fix here is to apply the item amount reduction before calling K_ThrowKartItem in any applicable cases, which makes the underflow not happen.

Fix applied:

ringracers0141

Attached is a reproduction script, 255repro.lua, which loads a test case where P1 gets their Orbinaut reflected by P2. It will automatically run after a second and can be repeated by calling the performsetup command in the developer console.

Changelog: Fixed an issue where a player could have 255 of an empty item in specific circumstances, which would prevent picking up Item Boxes or dropped items.