fread.c: warning: variable 'resj' set but not used

[mattdowle]

Noticed in clang-15 output, #5491 (comment), but maybe different options are in use now too. It's not occurring on CRAN so maybe this block has changed in dev, or the compiler version / options are different on CRAN. Anyway, the warning looks correct and useful.

fread.c:2539:21: warning: variable 'resj' set but not used [-Wunused-but-set-variable]
      for (int j=0, resj=-1; j<ncol; j++) {