pgsql: Check get_tle_by_resno() result before deref

Check get_tle_by_resno() result before deref

When creating a sort to support a group by, we need to look up the
target entry in the target list by the resno using get_tle_by_resno().
This particular code-path didn't check the result prior to attempting
to dereference it, while all other callers did.  While I can't see a
way for this usage of get_tle_by_resno() to fail (you can't ask for
a column to be sorted on which isn't included in the group by), it's
probably best to check that we didn't end up with a NULL somehow
anyway than risk the segfault.

I'm willing to back-patch this if others feel it's necessary, but my
guess is new features are what might tickle this rather than anything
existing.

Missing check spotted by the Coverity scanner.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/4ed22e891f9915b02b753ee8763a8f2438234fc6

Modified Files
--------------
src/backend/optimizer/plan/createplan.c |    3 +++
1 file changed, 3 insertions(+)