Inventory UI fixes

[BenjaminAmos]

Description

This pull request fixes various issues encountered when testing the inventory UI a bit more thoroughly:

• Fixes a crash when dropping the player's shield.
• Fixes a crash when using keyboard navigation after all items are sold.
• Changes highlight behaviour to target the nearest item after the currently highlighted item is removed.

Testing

• Start a new game
• Open the inventory screen and drop each of your items one-by-one.
• When an item is dropped, the highlight cursor should move to the next item along (or the previous item, for the last item).
• The game should not crash when you drop your shield.
• Additionally, follow the instructions in list highlight crashing program #711 and ensure that the game does not crash.

Notes

This fixes #711.