UBIFS: remove douple initialization in change_category()

"heap" is initialized twice.  I removed the first one, because it makes
Smatch complain that we use "new_cat" as an offset before checking it.

This doesn't change how the code works, it's just a cleanup.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>