Curved portions are formed on a first vertical wall portion that continues from a headwall portion and curved portions are formed on a second vertical wall portion that continues from the headwall portion. A difference in the linear length on the first vertical wall portion side before forming and after forming is cancelled out (i.e., absorbed) by the sum of the amounts that the curved portions protrude. Similarly, a difference in the linear length on the second vertical wall portion side before forming and after forming is cancelled out (i.e., absorbed) by the sum of the amounts that the curved portions protrude. As a result, tensile stress on the first vertical wall portion side and compression stress on the second vertical wall portion side can be prevented or minimized.