oleform: incorrect parsing of some captions

[decalage2]

Since PR #532, it seems that oleform parsing is slightly incorrect in some cases:
Looking at the results from https://github.com/decalage2/oletools/blob/master/tests/oleform/test_basic.py, caption strings and other fields are off by a few bytes.

For example for this CheckBox1, the caption should be "mouahaha":

But oleform returns this:

             ('word/vbaProject.bin',
              u'UserFormTEST1',
              {'ClsidCacheIndex': 26,
               'caption': '\xba\xa5\x18mouah',
               'control_tip_text': None,
               'group_name': '',
               'id': 5,
               'name': 'CheckBox1',
               'tabindex': 4,
               'tag': None,
               'value': '1'}),