fixes for durability step and prevItemId check

1. When deciding which durability skill to use, if you don't have
Master's Mend II but are 50+ durability away from max, it would choose
nothing and later error because bestSkill was nil.

2. When checking if it made an item before (to reuse progress per step
info, etc) it was erroneously checking an undefined local "prevItemId"
instead of "CraftingTool.prevItemId".
Merge pull request #1 from jre2/master

fixes for durability step and prevItemId check