Cast to long and add test/guard.

[JimBobSquarePants]

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Description

Fix #1167

[codecov]

Codecov Report

Merging #1180 into master will decrease coverage by 0.00%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #1180      +/-   ##
==========================================
- Coverage   82.47%   82.47%   -0.01%     
==========================================
  Files         690      690              
  Lines       29916    29918       +2     
  Branches     3379     3380       +1     
==========================================
  Hits        24674    24674              
- Misses       4541     4542       +1     
- Partials      701      702       +1     

Flag	Coverage Δ
#unittests	82.47% <50.00%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
...mory/DiscontiguousBuffers/MemoryGroupExtensions.cs	89.32% <0.00%> (-1.77%)	⬇️
src/ImageSharp/Memory/Buffer2D{T}.cs	93.75% <100.00%> (ø)

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[antonfirsov]

LGTM, thanks!
Seems too expensive to properly test this corner case on Buffer2D<T>.