estimate_path_cost_size() failed to re-use cached costs when the cached
startup/total cost was 0, so it calculated the costs redundantly.

This is an oversight in commit aa09cd24; but apply the patch to HEAD
only because there are no reports of actual trouble from that.

Author: Etsuro Fujita
Discussion: https://postgr.es/m/5C4AF3F3.4060409%40lab.ntt.co.jp