pgsql: Fix possible usage of incorrect UPPERREL_SETOP RelOptInfo

Fix possible usage of incorrect UPPERREL_SETOP RelOptInfo

03d40e4b5 allowed dummy UNION [ALL] children to be removed from the plan
by checking for is_dummy_rel(). That commit neglected to still account
for the relids from the dummy rel so that the correct UPPERREL_SETOP
RelOptInfo could be found and used for adding the Paths to.

Not doing this could result in processing of subsequent UNIONs using the
same RelOptInfo as a previously processed UNION, which could result in
add_path() freeing old Paths that are needed by the previous UNION.

The same fix was independently submitted (2 mins later) by Richard Guo.

Reported-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Author: David Rowley <dgrowleyml(at)gmail(dot)com>
Discussion: https://postgr.es/m/bee34aec-659c-46f1-9ab7-7bbae0b7616c@gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/fdda78e361f136ec2b8de579b366c1e66bba1199

Modified Files
--------------
src/backend/optimizer/prep/prepunion.c | 8 ++++++--
src/test/regress/expected/union.out | 6 +++---
2 files changed, 9 insertions(+), 5 deletions(-)