Backport of the fix for bug#25421 to 5.0.

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

@@ -6066,6 +6066,9 @@ 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 &&