pgsql: Check for stack overflow in transformSetOperationTree().

Check for stack overflow in transformSetOperationTree().

Since transformSetOperationTree() recurses, it can be driven to stack
overflow with enough UNION/INTERSECT/EXCEPT clauses in a query.  Add a
check to ensure it fails cleanly instead of crashing.  Per report from
Matthew Gerber (though it's not clear whether this is the only thing
going wrong for him).

Historical note: I think the reasoning behind not putting a check here in
the beginning was that the check in transformExpr() ought to be sufficient
to guard the whole parser.  However, because transformSetOperationTree()
recurses all the way to the bottom of the set-operation tree before doing
any analysis of the statement's expressions, that check doesn't save it.

Branch
------
REL8_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/2faab1af69de2a04bf98e79638393493c4867949

Modified Files
--------------
src/backend/parser/analyze.c |    4 ++++
1 files changed, 4 insertions(+), 0 deletions(-)