[C] Set current recipe to null when a recipe isn't found.  Fixes BUKKIT-2311

**The Issue:**
CraftItemEvent triggers when you click the empy result slot only after you crafted something in it, even if you have or not a result or ingredients, it still triggers and getRecipe() returns the last recipe because there isn't a "current" one.

This issue was fixed in 1.2.5 but during various updates since then, the patch was lost, causing the issue to resurface.

**PR Breakdown:**
This commit simply reimplements a fix that was already made but lost in the update process.

**Ticket:**
https://bukkit.atlassian.net/browse/BUKKIT-2311