R_DATATABLE_NUM_THREADS env variable limit fixed

[mattdowle]

Closes #4514

[codecov]

Codecov Report

Merging #4562 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4562   +/-   ##
=======================================
  Coverage   99.61%   99.61%           
=======================================
  Files          73       73           
  Lines       14111    14112    +1     
=======================================
+ Hits        14056    14057    +1     
  Misses         55       55           

Impacted Files	Coverage Δ
src/openmp-utils.c	100.00% <100.00%> (ø)

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