Update binarization_util.py

[HarshineeSriram]

In the BinBinarizer.fit() method, the line tmp_num_thresholds -= 1 is called before tmp_num_thresholds is initialized.

Hence, if you call the BinBinarizer.fit() method, you can run into this issue: UnboundLocalError: local variable 'tmp_num_thresholds' referenced before assignment.