UltraRogue: clear the next_obj field when removing items from the floor. The next_obj field is a pointer which the top item in a stack uses to keep a list of the other items. When removing an item from the stack, rem_obj() failed to set next_obj to NULL, which can cause items in monster inventory to point to items earlier in the inventory list. That causes infinite co-recursion when saving or restoring.

182e26224f92e874cbcf17d96fe2f5cee8bda7d7 rel2016.06