Fix a crash that occurs when the number of fields is small

[yaslab]

This PR resolves #83.

[codecov]

Codecov Report

Merging #92 into master will increase coverage by 0.29%.
The diff coverage is 97.82%.

@@            Coverage Diff             @@
##           master      #92      +/-   ##
==========================================
+ Coverage   90.63%   90.93%   +0.29%     
==========================================
  Files          14       14              
  Lines        2093     2128      +35     
==========================================
+ Hits         1897     1935      +38     
+ Misses        196      193       -3

Impacted Files	Coverage Δ
Tests/CSVTests/CSVReaderTests.swift	100% <100%> (ø)	⬆️
Sources/CSV/CSVReader.swift	93.67% <100%> (-0.15%)	⬇️
Tests/CSVTests/CSVRowDecoderTests.swift	92.6% <94.44%> (+0.08%)	⬆️
Sources/CSV/CSVRowDecoder.swift	74.35% <0%> (+0.93%)	⬆️

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