pgsql: Fix possible crash in partition-wise join.

Fix possible crash in partition-wise join.

The previous code assumed that we'd always succeed in creating
child-joins for a joinrel for which partition-wise join was considered,
but that's not guaranteed, at least in the case where dummy rels
are involved.

Ashutosh Bapat, with some wordsmithing by me.

Discussion: http://postgr.es/m/CAFjFpRf8=uyMYYfeTBjWDMs1tR5t--FgOe2vKZPULxxdYQ4RNw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f069c91a5793ff6b7884120de748b2005ee7756f

Modified Files
--------------
src/backend/optimizer/path/allpaths.c | 18 +++---------
src/backend/optimizer/path/joinrels.c | 16 -----------
src/backend/optimizer/util/relnode.c | 5 +++-
src/include/nodes/relation.h | 14 +++++----
src/test/regress/expected/partition_join.out | 43 ++++++++++++++++------------
src/test/regress/sql/partition_join.sql | 2 +-
6 files changed, 43 insertions(+), 55 deletions(-)