[C] Fix CraftInventoryCrafting#getMatrix returning array length. Fixes BUKKIT-4877

### The Issue:
When CraftInventoryCrafting#getMatrix() is called, it returns an array with 10 or 5 elements, while the matrix inventory has 9 or 4 elements, that are put into the array.
Therefore the array is one element too long.

### Justification for this PR:
The JIRA Report has been closed for "No explanation as to why this isn't intended behavior.".
Explaining it did not work and the bug is still, almost half a year, present in CraftBukkit.

### PR Breakdown:
The returned array will no longer initialized with the size of the whole inventory but rather with the size of the matrix inventory.

### Testing Results and Materials:
The change is trivial. 
I'm sorry, I really can't provide a test for such a small bug, that should be obvious when you look at the code.

### JIRA Ticket:
BUKKIT-4877- https://bukkit.atlassian.net/browse/BUKKIT-4877