fix logic error in fill_buffer

[ZoeyR]

Closes #335

[codecov-commenter]

Codecov Report

Merging #336 into master will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #336      +/-   ##
==========================================
- Coverage   61.42%   61.40%   -0.02%     
==========================================
  Files          11       11              
  Lines         827      824       -3     
==========================================
- Hits          508      506       -2     
+ Misses        319      318       -1     

Impacted Files	Coverage Δ
src/lib.rs	75.00% <ø> (ø)
src/de/read.rs	53.84% <100.00%> (-0.70%)	⬇️

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[dtolnay]

Implementation looks fine. Is there a way to add a test with a Deserialize impl for which this makes a difference? It's weird that this wouldn't have been covered by the test suite.

[ZoeyR]

I can design a small unit test that makes sure this function behaves properly. The test suite probably doesn't have complex enough types to trigger this.

[dtolnay]

Thanks!