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

[omjavaid]

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.

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

def test_unique_stacks(self):
         self.assertTrue(
             num_threads >= 10,
             'Number of expected threads and actual threads do not match.')
 
         # Attempt to walk each of the thread's executing the thread3 function to
         # 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
 
         expect_threads = ""
         for i in range(num_threads):
             thread = process.GetThreadAtIndex(i)
             self.assertTrue(thread.IsValid())
             if not is_thread3(thread):