Use read number of items instead of raw array size in load_names

The load_names benchmark expected the input CSV with domains would fill
the whole item array and it would crash when the number of lines would
be less than that.

Fix the expectations by using the real number or lines read to calculate
the array start and end position for each benchmark thread.

tests/bench/load-names.c

@@ -552,7 +552,7 @@ main(int argc, char *argv[]) {
 			isc_mem_create(&mem);
 			map = fun->new (mem);
 
-			size_t nitems = ARRAY_SIZE(item) / (nthreads + 1);
+			size_t nitems = lines / (nthreads + 1);
 
 			isc_barrier_init(&barrier, nthreads);