Bug: Crash during startup

[Jonas Höchst]

python3 -m radiotracking -s 250000 -b 25000 -d 0 1 2 3

Traceback (most recent call last):
  File "_ctypes/callbacks.c", line 232, in 'calling callback function'
  File "/usr/local/lib/python3.7/dist-packages/rtlsdr/rtlsdr.py", line 612, in _bytes_converter_callback
    self._callback_bytes(values, context)
  File "/usr/local/lib/python3.7/dist-packages/rtlsdr/rtlsdr.py", line 659, in _samples_converter_callback
    self._callback_samples(iq, context)
  File "/home/pi/pyradiotracking/radiotracking/analyze.py", line 78, in process_samples
    signals = self.extract_signals(freqs, times, spectrogram, ts_start)
  File "/home/pi/pyradiotracking/radiotracking/analyze.py", line 154, in extract_signals
    power = self._spectrogram_last[fi, start]
IndexError: index -5 is out of bounds for axis 1 with size 4