Improve handling NULL characters

[kiwiroy]

AFAICT the following is making for an "infinite" loop (nrowLimit is very large by default).

data.table/src/fread.c

Line 2151 in 0dfb91d

if (*tch=='\0') continue; // empty last line

Adding nrowLimit = myNrow to the branch appears to be a fix to force it to be the last line.

References

#3400

[codecov]

Codecov Report

Merging #3433 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3433      +/-   ##
==========================================
+ Coverage   95.06%   95.06%   +<.01%     
==========================================
  Files          65       65              
  Lines       12249    12251       +2     
==========================================
+ Hits        11644    11646       +2     
  Misses        605      605

Impacted Files	Coverage Δ
src/fread.c	98.34% <100%> (ø)	⬆️

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[codecov]

Codecov Report

Merging #3433 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3433      +/-   ##
==========================================
+ Coverage   96.65%   96.66%   +<.01%     
==========================================
  Files          65       65              
  Lines       12275    12277       +2     
==========================================
+ Hits        11865    11867       +2     
  Misses        410      410

Impacted Files	Coverage Δ
src/fread.c	98.5% <100%> (ø)	⬆️

---

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Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[mattdowle]

Thanks @kiwiroy! I've invited you to be project member.