pgsql: Avoid core dump in _outPathInfo() for Path without a parent RelO

Avoid core dump in _outPathInfo() for Path without a parent RelOptInfo.

Nearly all Paths have parents, but a ResultPath representing an empty FROM
clause does not.  Avoid a core dump in such cases.  I believe this is only
a hazard for debugging usage, not for production, else we'd have heard
about it before.  Nonetheless, back-patch to 9.1 where the troublesome code
was introduced.  Noted while poking at bug #11703.

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/96139cfbbc1581d6f17b601b37ce9dda88d36322

Modified Files
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src/backend/nodes/outfuncs.c |    5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)