Occasional crash in add_category_labels.py

[mshvartsman]

This one seems to come and go, have not yet tracked it down and want to document it before I head to bed. string_rows seems empty, not sure why. Might be a bug in fill_in_zeros().

Traceback (most recent call last):
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/pdb.py", line 1314, in main
    pdb._runscript(mainpyfile)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/pdb.py", line 1233, in _runscript
    self.run(statement)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/bdb.py", line 387, in run
    exec cmd in globals, locals
  File "<string>", line 1, in <module>
  File "add_category_labels.py", line 196, in <module>
    updated_data = update_database(clean_data)
  File "add_category_labels.py", line 188, in update_database
    joined_data.append(dict(d.items() + cat_rows[i].items() + string_rows[i].items()))
IndexError: list index out of range