dogroups: resize missing groups to one row

Author: aitap

src/dogroups.c

@@ -217,6 +217,7 @@ SEXP dogroups(SEXP dt, SEXP dtcols, SEXP groups, SEXP grpcols, SEXP jiscols, SEX
     }
     if (istarts[i] == NA_INTEGER || (LENGTH(order) && iorder[ istarts[i]-1 ]==NA_INTEGER)) {
       for (int j=0; j<length(SDall); ++j) {
+        SETLENGTH(VECTOR_ELT(SDall, j), 1);
         writeNA(VECTOR_ELT(SDall, j), 0, 1, false);
         // writeNA uses SET_ for STR and VEC, and we always use SET_ to assign to SDall always too. Otherwise,
         // this writeNA could decrement the reference for the old value which wasn't incremented in the first place.