fix(tar): clear rest of buffer when eof is reached

[piksel]

When reading a tar file using TarBuffer and the EOF was reached before a full record could be read, the ReadRecordAsync method still treats it as a successful read, by assuming the rest of the buffer just contained zeros.

This is not necessarily the case with a SharedArray buffer, which it currently uses. This PR makes sure that the rest of the buffer is cleared before returning.

Fixes #782.

[codecov]

Codecov Report

Merging #789 (12cc7be) into master (6c96ce2) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #789      +/-   ##
==========================================
- Coverage   74.71%   74.71%   -0.01%     
==========================================
  Files          72       72              
  Lines        8465     8468       +3     
==========================================
+ Hits         6325     6327       +2     
- Misses       2140     2141       +1     

Impacted Files	Coverage Δ
src/ICSharpCode.SharpZipLib/Tar/TarBuffer.cs	73.61% <100.00%> (+0.49%)	⬆️
...Code.SharpZipLib/Zip/Compression/DeflaterEngine.cs	85.66% <0.00%> (-0.38%)	⬇️

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