fix System.ArgumentOutOfRangeException issue in CustomStopWordsRemovingTransformer

[frank-dong-ms-zz]

fix issue #4518

[codecov]

Codecov Report

Merging #4592 into master will increase coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #4592      +/-   ##
==========================================
+ Coverage   75.12%   75.14%   +0.01%     
==========================================
  Files         913      913              
  Lines      160855   160855              
  Branches    17303    17303              
==========================================
+ Hits       120846   120868      +22     
+ Misses      35164    35139      -25     
- Partials     4845     4848       +3

Flag	Coverage Δ
#Debug	75.14% <0%> (+0.01%)	⬆️
#production	70.53% <0%> (+0.02%)	⬆️
#test	90.29% <ø> (-0.01%)	⬇️

Impacted Files	Coverage Δ
...ML.Transforms/Text/StopWordsRemovingTransformer.cs	86.1% <0%> (-0.16%)	⬇️
...soft.ML.TestFramework/DataPipe/TestDataPipeBase.cs	76.08% <0%> (-0.4%)	⬇️
src/Microsoft.ML.Transforms/Text/LdaTransform.cs	84.44% <0%> (-0.16%)	⬇️
...rosoft.ML.AutoML/ColumnInference/TextFileSample.cs	62.25% <0%> (+2.64%)	⬆️
....ML.AutoML/PipelineSuggesters/PipelineSuggester.cs	83.19% <0%> (+3.36%)	⬆️
src/Microsoft.ML.Core/Data/ProgressReporter.cs	77.94% <0%> (+6.98%)	⬆️