Fix processing of multi-stream files

[gfrontera]

Fixes issue #86

[gfrontera]

Perhaps there is a better fix but this patch fixes the problem for me.

[jeking3]

Two things:

1. Is there a test that reasonably exercised this? I'm thinking no, since the original code was not failing tests, so we could use a test.

2. Rebase on develop and force push so it builds clean with CI fixes. Thanks.

[jeking3]

We could use a test with input that leverages multi-stream files.

[codecov]

Codecov Report

Merging #87 into develop will decrease coverage by 0.09%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           develop      #87     +/-   ##
==========================================
- Coverage    69.04%   68.94%   -0.1%     
==========================================
  Files           80       80             
  Lines         3437     3439      +2     
  Branches      1025     1027      +2     
==========================================
- Hits          2373     2371      -2     
- Misses         451      452      +1     
- Partials       613      616      +3

Impacted Files	Coverage Δ
include/boost/iostreams/filter/bzip2.hpp	70.27% <93.33%> (-0.57%)	⬇️
src/bzip2.cpp	84.9% <0%> (-5.67%)	⬇️

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[gfrontera]

Thank you for your feedback, @jeking3.
It turns out a test did exist. However, the size of the compressed multi-stream was too small for the bug to manifest. The whole multi-stream could fit in the buffer at the same time, and in that case the existing implementation worked correctly.
I have just increased the size of the compressed multi-stream (approximately 5x), so now the test fails without this proposed fix.

[gfrontera]

Please @jeking3, could you re-review this pull request to see if the changes made are sufficient?

[jeking3]

Looks like it now:

meet the postcondition requiring that either i1 == i2 or o1 == o2

described in the related issue.