Fix a crash bug that would occur if the player had autodrop on

Crash reproduction steps (prior to this fix): make sure autodrop is set to "1". Find two full cans. Drink from one can, then immediately drink from the other.

I suspect that it's *any* time you have only one can in your inventory, but didn't test that before fixing. The original instructions I got were to drink a can while standing on top of a can or bottle.

Thanks to wad67_linux on IRC for the bug report and repro steps!