Fix EXIF overflow and Jpeg decoding

[JimBobSquarePants]

Prerequisites

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• I have provided test coverage for my change (where applicable)

Description

This PR fixes the overflow error in #694 and #696
Additionally is allows the decoding of the malformed jpeg in #695

[codecov]

Codecov Report

Merging #699 into master will increase coverage by <.01%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #699      +/-   ##
==========================================
+ Coverage   89.25%   89.26%   +<.01%     
==========================================
  Files         878      878              
  Lines       39240    39246       +6     
  Branches     2555     2561       +6     
==========================================
+ Hits        35025    35033       +8     
+ Misses       3492     3487       -5     
- Partials      723      726       +3

Impacted Files	Coverage Δ
tests/ImageSharp.Tests/TestImages.cs	100% <ø> (ø)	⬆️
...Sharp.Tests/Formats/Jpg/JpegDecoderTests.Images.cs	100% <100%> (ø)	⬆️
...s/ImageSharp.Tests/Formats/Jpg/JpegDecoderTests.cs	56.45% <100%> (+1.45%)	⬆️
src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs	84.93% <66.66%> (+0.46%)	⬆️
...rc/ImageSharp/MetaData/Profiles/Exif/ExifReader.cs	69.6% <87.5%> (+0.3%)	⬆️

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