Prevent segfaults when item ID > max ID in Equip menu

This prevents segfaults (<https://github.com/EasyRPG/Player/issues/337>),
but the items are still handled incorrectly. Currently, such items give the
user 999 stats, but in RPG_RT they are handled as no items.
Check for item ID > max ID in game_actor

This prevents 999 stats bug which affected the battle.
Merge pull request #338 from rohkea/master

Prevent segfaults when item ID > max ID in Equip menu