pgsql: Make ExecGetInsertedCols() and friends more robust and improve c

Make ExecGetInsertedCols() and friends more robust and improve comments.

If ExecGetInsertedCols(), ExecGetUpdatedCols() or ExecGetExtraUpdatedCols()
were called with a ResultRelInfo that's not in the range table and isn't a
partition routing target, the functions would dereference a NULL pointer,
relinfo->ri_RootResultRelInfo. Such ResultRelInfos are created when firing
RI triggers in tables that are not modified directly. None of the current
callers of these functions pass such relations, so this isn't a live bug,
but let's make them more robust.

Also update comment in ResultRelInfo; after commit 6214e2b228,
ri_RangeTableIndex is zero for ResultRelInfos created for partition tuple
routing.

Noted by Coverity. Backpatch down to v11, like commit 6214e2b228.

Reviewed-by: Tom Lane, Amit Langote

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6e437ce5a6b64c443cf7c907db0988bf571e6042

Modified Files
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src/backend/executor/execUtils.c | 24 ++++++++++++++++++------
1 file changed, 18 insertions(+), 6 deletions(-)