postgres_fdw: Fix test for cached costs in estimate_path_cost_size().
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
aa09cd242f; 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