pgsql: Harden range_table_mutator() against null RangeTblEntry.subquery

Harden range_table_mutator() against null RangeTblEntry.subquery.

Commit 64919aaab made pull_up_simple_subquery set rte->subquery = NULL
after doing the deed, so that we don't waste cycles copying a
now-useless subquery tree around. This turns out to create a core dump
hazard in range_table_mutator, which supposes that that field is never
NULL. Apparently none of our own code invokes query_tree_mutator or
range_table_mutator on the top Query after subquery pullup; but it
wouldn't be surprising if outside code does, and anyway I'm working
on a v16 patch that will need it.

We can fix this cleanly by just getting rid of the special-case
handling of this field and treating it more like all the rest.
I think the special case might be left over from a time when
QTW_DONT_COPY_QUERY was the default behavior, but that was eons ago.

Thanks to Dean Rasheed for review.

Discussion: https://postgr.es/m/545569.1656107045@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ebc584ed495d7ad3a3f33db4e75d9543ff6a87d1

Modified Files
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src/backend/nodes/nodeFuncs.c | 16 ++++------------
1 file changed, 4 insertions(+), 12 deletions(-)