MDEV-10665: Json_writer produces extra members in output

Fix an issue in Single_line_formatting_helper: flush_on_one_line()
didn't clean up the buffered items which could cause them to be
printed for the second time.

This can't be ever observed by a user (see MDEV text for details).

sql/my_json_writer.cc

@@ -330,6 +330,8 @@ void Single_line_formatting_helper::flush_on_one_line()
     ptr++;
   }
   owner->output.append(']');
+  /* We've printed out the contents of the buffer, mark it as empty */
+  buf_ptr= buffer;
 }