pgsql: Fix possible crash in add_paths_to_append_rel()

Fix possible crash in add_paths_to_append_rel()

While working on a8a968a82, I failed to consider that
cheapest_startup_path can be NULL when there is no non-parameterized
path in the pathlist. This is well documented in set_cheapest(), I just
failed to notice.

Here we adjust the code to just check if the RelOptInfo has a
cheapest_startup_path set before adding it to the startup_subpaths list.

Reported-by: Richard Guo
Author: Richard Guo
Discussion: https://postgr.es/m/CAMbWs49w3t03V69XhdCuw+GDwivny4uQUxrkVp6Gejaspt0wMQ@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/fc4089f3c65a5f1b413a3299ba02b66a8e5e37d0

Modified Files
--------------
src/backend/optimizer/path/allpaths.c | 11 +++++++----
src/test/regress/expected/union.out | 16 ++++++++++++++++
src/test/regress/sql/union.sql | 9 ++++++++-
3 files changed, 31 insertions(+), 5 deletions(-)