Improve performance in _find_record_by_extent.
Improve performance in _find_record_by_extent.

Use _extent_location instead of extent_location, which
skips a check.

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

clalancette/pyiso,clalancette/pycdlib