dfxml.py: Add fill property to byte_run class

[ajnelson-nist]

In PR 22, one of the attempted fixes for an issue in has_sector caused byte runs to no longer be parsed correctly. Initializing self.fill = None in byte_run.__init__ caused self.len to be None for unknown reasons.

Steps to reproduce:

• Check out this commit.
• Run iblkfind.py on nps-2008-jean.

[simsong]

Is this still an issue? Can we reproduce it with the current version, rather than a previous commit?