Png Decoder Bounds check. Fix #849

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

Perform an additional bounds check in the ZlibInflateStream to handle odd chunks.

[codecov]

Codecov Report

Merging #851 into master will decrease coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #851      +/-   ##
==========================================
- Coverage   88.89%   88.79%   -0.11%     
==========================================
  Files        1014     1014              
  Lines       44289    44295       +6     
  Branches     3207     3208       +1     
==========================================
- Hits        39370    39331      -39     
- Misses       4198     4203       +5     
- Partials      721      761      +40

Impacted Files	Coverage Δ
...ts/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs	100% <ø> (ø)	⬆️
tests/ImageSharp.Tests/TestImages.cs	100% <ø> (ø)	⬆️
...c/ImageSharp/Formats/Png/Zlib/ZlibInflateStream.cs	55.38% <100%> (+2.15%)	⬆️
...xelFormats/PixelOperations{TPixel}.PixelBenders.cs	89.34% <0%> (-9.84%)	⬇️
...Processing/Processors/Transforms/TransformUtils.cs	88.29% <0%> (-5.32%)	⬇️
src/ImageSharp/Common/Helpers/ImageMaths.cs	82.05% <0%> (-5.13%)	⬇️
.../Processing/Processors/Transforms/CropProcessor.cs	96% <0%> (-4%)	⬇️
...ocessing/Processors/Dithering/ErrorDiffuserBase.cs	93.75% <0%> (-3.13%)	⬇️
...rs/Normalization/HistogramEqualizationProcessor.cs	97.5% <0%> (-2.5%)	⬇️
...ormats/Jpeg/Components/Decoder/QualityEvaluator.cs	94.04% <0%> (-2.39%)	⬇️
... and 18 more

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