Allow non-zero data in the third DR field.
Allow non-zero data in the third DR field.

There are ISOs in the wild that have data in there, even
though it violates the standard.

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

clalancette/pyiso,clalancette/pycdlib