Fix error in TestNumThreads.py when frame.GetFunctionName returns none

Summary:
This patch fixes an error happening in TestNumThreads.py when it encounters frame.GetFunctionName none for address only locations in stripped libc.

This error was showing up on arm-linux docker container running lldb buildbot.

Reviewers: labath

Reviewed By: labath

Subscribers: kristof.beyls, lldb-commits

Differential Revision: https://reviews.llvm.org/D79777