fread quote="" when last line too short and fill=TRUE

[mattdowle]

Crash when last line is too short and filled with fill=TRUE, and quote="" (ignore quoting) too. The final \0 at eof was being treated as a quote.
Reported by Jiucang Hao on email so there is no issue.

[codecov]

Codecov Report

Merging #3524 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3524      +/-   ##
==========================================
+ Coverage      97%   97.01%   +<.01%     
==========================================
  Files          66       66              
  Lines       12474    12475       +1     
==========================================
+ Hits        12101    12102       +1     
  Misses        373      373

Impacted Files	Coverage Δ
src/fread.c	98.49% <100%> (ø)	⬆️

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