fread nul in the middle of a field too

[mattdowle]

Follow up to #3505 and #3433 for #3400.
Thanks to a report and example file from Bingjie Qian received privately. (Name used with permission.)

[codecov]

Codecov Report

Merging #3514 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3514      +/-   ##
==========================================
+ Coverage   96.72%   96.72%   +<.01%     
==========================================
  Files          66       66              
  Lines       12424    12437      +13     
==========================================
+ Hits        12017    12030      +13     
  Misses        407      407

Impacted Files	Coverage Δ
src/freadR.c	96.28% <100%> (+0.12%)	⬆️
src/fread.c	98.49% <100%> (ø)	⬆️
src/nafill.c	100% <100%> (ø)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[codecov]

Codecov Report

Merging #3514 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3514      +/-   ##
==========================================
+ Coverage   96.72%   96.72%   +<.01%     
==========================================
  Files          66       66              
  Lines       12424    12434      +10     
==========================================
+ Hits        12017    12027      +10     
  Misses        407      407

Impacted Files	Coverage Δ
src/freadR.c	96.28% <100%> (+0.12%)	⬆️
src/fread.c	98.49% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 280226b...f4c8c03. Read the comment docs.