Return early in setnafill to avoid accessing 0-row element

[MichaelChirico]

Closes #7910.

The illegal access comes from here (ditto other input types, and "nocb"):

data.table/src/nafill.c

Line 26 in a3eeb4c

if (ISNAN(x[0]) && limit>0) { ans->dbl_v[0] = fill; fills = 1; }

This doesn't (currently) happen for type="const" because all accesses are in the trivial for loop there:

data.table/src/nafill.c

Line 10 in a3eeb4c

for (uint_fast64_t i=0; i<nx; i++) {

Returning up-front is good for efficiency anyway, so don't bother trying to avoid those accesses -- just quit once 0-row input is detected.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.02%. Comparing base (e26cf1b) to head (bcb810f).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7911   +/-   ##
=======================================
  Coverage   99.02%   99.02%           
=======================================
  Files          88       88           
  Lines       17376    17378    +2     
=======================================
+ Hits        17206    17208    +2     
  Misses        170      170           

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