[GLIBCXX_DEBUG] Don't decrement past begin of iterators during crafting.

To reproduce:
* Change DEBUG line in makefile to "DEBUG = -g -D_GLIBCXX_DEBUG"
* Recompile.
* Pick up two rocks and a stick.
* (optional) Unwield stick
* Try to craft a hammer.
* Hit two crashes in a row as it tries to consume the stick and then the first of two rocks.

Notes:
* Please double check my comparisons. I am not 100% on the inventory structure and I was guessing as to where to get the begin iterators for comparison. "It worked for me."