[Hotfix] solution for issue with v1.8.7

[jbygdell]

v1.8.7 introduced a bug where decryption would deadlock if the available read buffer was 0. Causing the stream to read the same byte over and over again and never reaching the end of the file.

This PR fixes the issue found in 1.8.7 and adds a decryption test for decrypting an entire file.

[pontus]

Thanks for the fix.

Having roughly the same code repeated for the different canRead cases looks a bit ugly to me, but we're in this mess because I didn't have them duplicated so I guess it's fine. (For reference; my fix that I didn't have time to fix proper tests for before was to add a check on buffer.Len() at the end of ensureBuffer and return io.EOF if it hadn't succeeded in ensuring the buffer had content to read, but either way is good I guess.

[pontus]

Looks good, see specific comment about stream position and return.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1ce976c) 66.29% compared to head (65b1a14) 66.46%.
Report is 2 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #106      +/-   ##
==========================================
+ Coverage   66.29%   66.46%   +0.17%     
==========================================
  Files           6        6              
  Lines        1178     1184       +6     
==========================================
+ Hits          781      787       +6     
  Misses        278      278              
  Partials      119      119              

Flag	Coverage Δ
unittests	66.46% <100.00%> (+0.17%)	⬆️

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