catch missing factor levels in rbindlist

[mattdowle]

Closes #3315

[MichaelChirico]

I would try this directly with structure instead of relying on diff, since the behavior of diff might change upstream

[codecov]

Codecov Report

Merging #3317 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #3317      +/-   ##
=========================================
+ Coverage    94.8%   94.8%   +<.01%     
=========================================
  Files          65      65              
  Lines       12094   12095       +1     
=========================================
+ Hits        11466   11467       +1     
  Misses        628     628

Impacted Files	Coverage Δ
src/rbindlist.c	88.86% <100%> (+0.02%)	⬆️

---

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 0e5fb5c...64062fc. Read the comment docs.

[codecov]

Codecov Report

Merging #3317 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #3317      +/-   ##
=========================================
+ Coverage    94.8%   94.8%   +<.01%     
=========================================
  Files          65      65              
  Lines       12094   12095       +1     
=========================================
+ Hits        11466   11467       +1     
  Misses        628     628

Impacted Files	Coverage Δ
src/rbindlist.c	88.86% <100%> (+0.02%)	⬆️

---

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 0e5fb5c...64062fc. Read the comment docs.