Fix two issues in Python code for parsing Orbit capture files
Fix two issues in Python code for parsing Orbit capture files

The fixed issues are:

1. Don't try to read the section list if it does not exist.
2. Use len(bytes) and not bytes.length (the latter does not exist).

Tested: Tested opening capture files where these cases occur.
Unit test.
google/orbit,google/orbit,google/orbit,google/orbit