SCUMM: Fix bug #6683, Loading a game saved during 'leadup' to a cutscene

When loading a game which was saved during a 'lead up' to a cutscene, _currentRoom is set to 0 (this occurs in MM/Zak/Indy3).

This started as of commit '973cb9a281e7b2dc73b7c640172954cc5f0bacfd' (27/04/2005), the new 'loadRoomSubBlocks' function began being used, which has a check to ensure a room resource was returned.