Adjusted Range_rowid_filter_cost_info lookup cost slightly.

If the array size would be 1, the cost would be 0 which is wrong.
Fixed by adding a small (0.001) base value to the lookup cost.

This causes not changes in any result files.

sql/rowid_filter.cc

@@ -29,7 +29,8 @@ lookup_cost(Rowid_filter_container_type cont_type)
 {
   switch (cont_type) {
   case SORTED_ARRAY_CONTAINER:
-    return log(est_elements)*0.01;
+    /* The addition is here to take care of arrays with 1 element */
+    return log(est_elements)*0.01+0.001;
   default:
     DBUG_ASSERT(0);
     return 0;