Fix CTTH composite not to mark invalid data as cloud-free

[mraspaud]

This PR fixes a bug that appears when the cloud height product is generated after resampling.

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

[codecov]

Codecov Report

Merging #661 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #661      +/-   ##
==========================================
- Coverage   78.55%   78.53%   -0.02%     
==========================================
  Files         138      138              
  Lines       20089    20089              
==========================================
- Hits        15781    15777       -4     
- Misses       4308     4312       +4

Impacted Files	Coverage Δ
satpy/tests/compositor_tests/__init__.py	99.32% <100%> (ø)	⬆️
satpy/composites/cloud_products.py	87.87% <100%> (ø)	⬆️
satpy/readers/__init__.py	93.33% <0%> (-1.22%)	⬇️

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[coveralls]

Coverage decreased (-0.02%) to 78.531% when pulling 5348689 on mraspaud:fix-cloud-products into 70cdacb on pytroll:master.