pgsql-server: Test HAVING condition before computing targetlist of an

Log Message:
-----------
Test HAVING condition before computing targetlist of an Aggregate node.
This is required by SQL spec to avoid failures in cases like
  SELECT sum(win)/sum(lose) FROM ... GROUP BY ... HAVING sum(lose) > 0;
AFAICT we have gotten this wrong since day one.  Kudos to Holger Jakobs
for being the first to notice.

Modified Files:
--------------
    pgsql-server/src/backend/executor:
        nodeAgg.c (r1.122 -> r1.123)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/executor/nodeAgg.c.diff?r1=1.122&r2=1.123)