Deal with a root SL entry properly.
Deal with a root SL entry properly.

In particular, if we run across a root entry, we want
to throw away the earlier portions of the name, and
just start again at the root.

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

clalancette/pyiso,clalancette/pycdlib