ipairs() called on enum attr tables never ends

To reproduce:
```
[lua]# @df.item_type
...
90                     	 = BRANCH
[lua]# @df.item_type.attrs
...
111655                 	 = <item_type._attr_entry_type: 0x7ffff7dcfca0>
...
```

This can be stopped with `kill-lua` if run with `dfhack-run` (since printall_ipairs() is pure Lua).

Reported by @wolfboyft on Discord