fix(zip): handle iterating updated entries in ZipInputStream

[piksel]

Currently ZipInputStream expects all archive entries to immediately follow the previous one. This is normally the case for archives created in one go, but necessarily true when they have been updated.
This PR allows ZipInputStream to skip any padding between entries and consequently read "direct-updated" archives.

Fixes #639

Curiously, 7-zip does not seem to handle these files either and just show all entries found before the first "padded" one. And of course, when updating it uses the equivalent of ZipFiles FileUpdateMode.Safe to avoid the issue.

[codecov]

Codecov Report

Merging #642 (77dbae4) into master (bdec777) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #642      +/-   ##
==========================================
+ Coverage   74.54%   74.60%   +0.05%     
==========================================
  Files          72       72              
  Lines        8450     8457       +7     
==========================================
+ Hits         6299     6309      +10     
+ Misses       2151     2148       -3     

Impacted Files	Coverage Δ
src/ICSharpCode.SharpZipLib/Zip/ZipInputStream.cs	82.46% <100.00%> (+2.07%)	⬆️

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