Safely erase savegame entries when they're inaccessable in shared mode.

Previous code would crash if you launched cataclysm with --shared and tried to load a save that didn't belong to you.
Now it correctly doesn't show other saves, even if there are none you own.

Use the returned iterator from erase instead of trying to work around it by decrementing.
Thanks for poklj and codehero for finding it.