Fix up parsing of Joliet descriptors.
Fix up parsing of Joliet descriptors.

This now actually does the wrong thing for new descriptors,
but we'll fix that shortly.

Signed-off-by: Chris Lalancette <281cd07d7578d97c83271fbbf2faddb83ab3791c@gmail.com>

clalancette/pycdlib,clalancette/pyiso