Crash when setting both min and max color to the same value in spectrogram

[sppmacd]

Reproduce:

Set both min and max color to the same value.

Traceback (most recent call last):
  File "friture/spectrogram.py", line 146, in handle_new_data
  File "friture/imageplot.py", line 225, in addData
  File "friture/imageplot.py", line 84, in addData
  File "friture/spectrogram_image.py", line 100, in addData
  File "friture/spectrogram_image.py", line 133, in floats_to_bytes
  File "friture/spectrogram_image.py", line 158, in color_from_float
  File "friture_extensions/lookup_table.pyx", line 35, in friture_extensions.lookup_table.pyx_color_from_float_2D
IndexError: Out of bounds on buffer access (axis 0)

[danny7989]

I've run into the same issue too.

Traceback (most recent call last):
  File "friture/spectrogram.py", line 146, in handle_new_data
  File "friture/imageplot.py", line 225, in addData
  File "friture/imageplot.py", line 84, in addData
  File "friture/spectrogram_image.py", line 100, in addData
  File "friture/spectrogram_image.py", line 133, in floats_to_bytes
  File "friture/spectrogram_image.py", line 158, in color_from_float
  File "friture_extensions/lookup_table.pyx", line 35, in friture_extensions.lookup_table.pyx_color_from_float_2D
IndexError: Out of bounds on buffer access (axis 0)

[thirstyice]

Duplicate of #80