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

lldb/test/API/functionalities/thread/num_threads/TestNumThreads.py

@@ -95,6 +95,8 @@ class NumberOfThreadsTestCase(TestBase):
         # the same breakpoint.
         def is_thread3(thread):
             for frame in thread:
+                if frame.GetFunctionName() is None:
+                    continue
                 if "thread3" in frame.GetFunctionName(): return True
             return False