Fix a bug if you have no roles at all
This could not happen currently, but might happen later on if a group
is defined that does not have an 'anybody' membership.
Patch by: Sverre Rabbelier
Coverage: code coverage testing for Python
Coverage.py is a Python module that measures code coverage during test execution.
It uses the code analysis tools and tracing hooks provided in the Python standard
library to determine which lines are executable, and which have been executed.
For more information, see http://nedbatchelder.com/code/modules/coverage.html