pgsql: Throw an error for negative LIMIT or OFFSET values, instead of

Log Message:
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Throw an error for negative LIMIT or OFFSET values, instead of silently
treating them as zero. Simon Riggs

Modified Files:
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pgsql/src/backend/executor:
nodeLimit.c (r1.33 -> r1.34)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeLimit.c?r1=1.33&r2=1.34)