pgsql: Fix mistakes in commit 4020b370f214315b8c10430301898ac21658143f

Fix mistakes in commit 4020b370f214315b8c10430301898ac21658143f

cost_tidrangescan() was setting the disabled_nodes value correctly,
and then immediately resetting it to zero, due to poor code editing on
my part.

materialized_finished_plan correctly set matpath.parent to
zero, but forgot to also set matpath.parallel_workers = 0, causing
an access to uninitialized memory in cost_material. (This shouldn't
result in any real problem, but it makes valgrind unhappy.)

reparameterize_path was dereferencing a variable before verifying that
it was not NULL.

Reported-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> (issue #1)
Reported-by: Michael Paquier <michael(at)paquier(dot)xyz> (issue #1)
Diagnosed-by: Lukas Fittl <lukas(at)fittl(dot)com> (issue #1)
Reported-by: Zsolt Parragi <zsolt(dot)parragi(at)percona(dot)com> (issue #2)
Reported-by: Richard Guo <guofenglinux(at)gmail(dot)com> (issue #3)
Discussion: http://postgr.es/m/CAN4CZFPvwjNJEZ_JT9Y67yR7C=KMNa=LNefOB8ZY7TKDcmAXOA@mail.gmail.com
Discussion: http://postgr.es/m/aXrnPgrq6Gggb5TG@paquier.xyz

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/71c1136989b363004357efb54c87b4192749a6a0

Modified Files
--------------
src/backend/optimizer/path/costsize.c | 1 -
src/backend/optimizer/plan/createplan.c | 5 ++++-
src/backend/optimizer/util/pathnode.c | 4 ++--
3 files changed, 6 insertions(+), 4 deletions(-)