PIT entry update crash

[s3lph]

I encountered the following stacktraces (on separate occasions), crashing the BasicICNLayer process:

Process Process-98:
Traceback (most recent call last):
  File "/usr/lib/python3.6/multiprocessing/process.py", line 258, in _bootstrap
    self.run()
  File "/usr/lib/python3.6/multiprocessing/process.py", line 93, in run
    self._target(*self._args, **self._kwargs)
  File ".../picn/PiCN/Processes/LayerProcess.py", line 144, in _run
    self._run_poll(from_lower, from_higher, to_lower, to_higher)
  File ".../picn/PiCN/Processes/LayerProcess.py", line 85, in _run_poll
    self.data_from_lower(to_lower, to_higher, from_lower.get())
  File ".../picn/PiCN/Layers/ICNLayer/BasicICNLayer.py", line 83, in data_from_lower
    self.handle_interest(face_id, packet, to_lower, to_higher, False)
  File ".../picn/PiCN/Layers/ICNLayer/BasicICNLayer.py", line 103, in handle_interest
    self.update_timestamp_in_pit(pit_entry)
  File ".../picn/PiCN/Layers/ICNLayer/BasicICNLayer.py", line 282, in update_timestamp_in_pit
    pit.update_timestamp(pit_entry)
  File ".../picn/PiCN/Layers/ICNLayer/PendingInterestTable/PendingInterestTableMemoryExact.py", line 40, in update_timestamp
    self._container.remove(pit_entry)
ValueError: list.remove(x): x not in list

Process Process-58:
Traceback (most recent call last):
  File "/usr/lib/python3.6/multiprocessing/process.py", line 258, in _bootstrap
    self.run()
  File "/usr/lib/python3.6/multiprocessing/process.py", line 93, in run
    self._target(*self._args, **self._kwargs)
  File ".../picn/PiCN/Processes/LayerProcess.py", line 144, in _run
    self._run_poll(from_lower, from_higher, to_lower, to_higher)
  File ".../picn/PiCN/Processes/LayerProcess.py", line 85, in _run_poll
    self.data_from_lower(to_lower, to_higher, from_lower.get())
  File ".../picn/PiCN/Layers/ICNLayer/BasicICNLayer.py", line 83, in data_from_lower
    self.handle_interest(face_id, packet, to_lower, to_higher, False)
  File ".../picn/PiCN/Layers/ICNLayer/BasicICNLayer.py", line 115, in handle_interest
    self.add_used_fib_entry_to_pit(interest.name, new_face_id)
  File ".../picn/PiCN/Layers/ICNLayer/BasicICNLayer.py", line 266, in add_used_fib_entry_to_pit
    pit.add_used_fib_entry(name, used_fib_entry)
  File ".../picn/PiCN/Layers/ICNLayer/PendingInterestTable/PendingInterestTableMemoryExact.py", line 47, in add_used_fib_entry
    self._container.remove(pit_entry)
ValueError: list.remove(x): x not in list