regression in NTSC --WibbleRemover

[Gamnn]

d26b826 seemed to break NTSC WibbleRemover.

Traceback (most recent call last):
  File "/home/gamn/github/ld-decode/ld-decode", line 78, in <module>
    ldd = LDdecode(filename, outname, loader, analog_audio = 0 if args.daa else 44.100, digital_audio = not args.noefm, system=system, doDOD = not args.nodod, threads=args.threads, extra_options=extra_options)
  File "/home/gamn/github/ld-decode/lddecode/core.py", line 2533, in __init__
    self.rf = RFDecode(system=system, decode_analog_audio=analog_audio, decode_digital_audio=digital_audio, has_analog_audio = self.has_analog_audio, extra_options = extra_options)
  File "/home/gamn/github/ld-decode/lddecode/core.py", line 338, in __init__
    self.computefilters()
  File "/home/gamn/github/ld-decode/lddecode/core.py", line 346, in computefilters
    self.computevideofilters()
  File "/home/gamn/github/ld-decode/lddecode/core.py", line 444, in computevideofilters
    video_notch = sps.butter(3, [DP['video_lpf_freq']/self.freq_half, 5.0/self.freq_half], 'bandstop')
  File "/home/gamn/.local/lib/python3.6/site-packages/scipy/signal/filter_design.py", line 2858, in butter
    output=output, ftype='butter', fs=fs)
  File "/home/gamn/.local/lib/python3.6/site-packages/scipy/signal/filter_design.py", line 2341, in iirfilter
    raise ValueError("Digital filter critical frequencies "
ValueError: Digital filter critical frequencies must be 0 < Wn < 1