Sanitize Gif descriptor bounds.

[JimBobSquarePants]

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Description

Fixes #1530

The provided test image contains descriptor information that generates a restore rectangle lager than the frame dimensions. The fix limits the maximum bounds to the frame dimensions.

[antonfirsov]

LGTM

[codecov]

Codecov Report

Merging #1532 (ba115e1) into master (7dfea13) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1532      +/-   ##
==========================================
+ Coverage   83.50%   83.52%   +0.01%     
==========================================
  Files         742      742              
  Lines       32801    32802       +1     
  Branches     3671     3671              
==========================================
+ Hits        27392    27399       +7     
+ Misses       4691     4689       -2     
+ Partials      718      714       -4     

Flag	Coverage Δ
unittests	83.52% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
src/ImageSharp/Formats/Gif/GifDecoderCore.cs	86.58% <100.00%> (+2.66%)	⬆️

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