Fix a regression introduced in #2329

[renkun-ken]

Closes #2338

[codecov-io]

Codecov Report

Merging #2342 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2342      +/-   ##
==========================================
+ Coverage   90.92%   91.01%   +0.08%     
==========================================
  Files          61       61              
  Lines       11808    11810       +2     
==========================================
+ Hits        10737    10749      +12     
+ Misses       1071     1061      -10

Impacted Files	Coverage Δ
R/data.table.R	97.42% <100%> (ø)	⬆️
src/rbindlist.c	88.74% <0%> (-0.2%)	⬇️
src/forder.c	94.47% <0%> (+0.52%)	⬆️
src/assign.c	94.36% <0%> (+1.23%)	⬆️

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