Fix for bug #25421: MySQL threads don't respond to the kill command

Calculating the estimated number of records for a range scan may take a
significant time, and it was impossible for a user to interrupt that
process by killing the connection or the query.

Fixed by checking the thread's 'killed' status in check_quick_keys() and
interrupting the calculation process if it is set to a non-zero value.

sql/opt_range.cc

@@ -7352,6 +7352,9 @@ check_quick_keys(PARAM *param, uint idx, SEL_ARG *key_tree,
     tmp_max_flag= max_key_flag | key_tree->max_flag;
   }
 
+  if (unlikely(param->thd->killed != 0))
+    return HA_POS_ERROR;
+  
   keynr=param->real_keynr[idx];
   param->range_count++;
   if (!tmp_min_flag && ! tmp_max_flag &&