stats stack output often crashes when failing to get the pc from a single thread

[sdimitro]

without even showing what the other threads are doing:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/sdb/internal/repl.py", line 107, in eval_cmd
    for obj in invoke([], input_):
  File "/usr/lib/python3/dist-packages/sdb/pipeline.py", line 152, in invoke
    yield from execute_pipeline(first_input, pipeline)
  File "/usr/lib/python3/dist-packages/sdb/pipeline.py", line 84, in execute_pipeline
    yield from massage_input_and_call(pipeline[-1], this_input)
  File "/usr/lib/python3/dist-packages/sdb/pipeline.py", line 67, in massage_input_and_call
    yield from cmd.call(objs)
  File "/usr/lib/python3/dist-packages/sdb/command.py", line 413, in call
    yield from self.__invalid_memory_objects_check(
  File "/usr/lib/python3/dist-packages/sdb/command.py", line 358, in __invalid_memory_objects_check
    for obj in objs:
  File "/usr/lib/python3/dist-packages/sdb/command.py", line 625, in _call
    self.pretty_print(self.caller(objs))
  File "/usr/lib/python3/dist-packages/sdb/commands/linux/stacks.py", line 407, in pretty_print
    self.print_stacks(filter(self.match_stack, objs))
  File "/usr/lib/python3/dist-packages/sdb/commands/linux/stacks.py", line 382, in print_stacks
    for stack_key, tasks in KernelStacks.aggregate_stacks(objs):
  File "/usr/lib/python3/dist-packages/sdb/commands/linux/stacks.py", line 376, in aggregate_stacks
    tuple(KernelStacks.get_frame_pcs(task)))
  File "/usr/lib/python3/dist-packages/sdb/commands/linux/stacks.py", line 236, in get_frame_pcs
    frame_pcs.append(frame.pc)