Cut radiances array depending on number of scans

[mraspaud]

This fixes the problem with reading compact viirs granules that have a lower number of scans.

• Tests passed
• Passes git diff origin/master **/*py | flake8 --diff

[codecov]

Codecov Report

Merging #340 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #340      +/-   ##
==========================================
- Coverage   69.13%   69.12%   -0.01%     
==========================================
  Files         123      123              
  Lines       15630    15632       +2     
==========================================
  Hits        10806    10806              
- Misses       4824     4826       +2

Impacted Files	Coverage Δ
satpy/readers/viirs_compact.py	13.87% <0%> (-0.12%)	⬇️

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