pgsql: Reject zero or negative BY step in plpgsql integer FOR-loops, and

Log Message:
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Reject zero or negative BY step in plpgsql integer FOR-loops, and behave
sanely if the loop value overflows int32 on the way to the end value.
Avoid useless computation of "SELECT 1" when BY is omitted. Avoid some
type-punning between Datum and int4 that dates from the original coding.

Modified Files:
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pgsql/src/pl/plpgsql/src:
gram.y (r1.102 -> r1.103)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y.diff?r1=1.102&r2=1.103)
pl_exec.c (r1.197 -> r1.198)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c.diff?r1=1.197&r2=1.198)
pl_funcs.c (r1.59 -> r1.60)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_funcs.c.diff?r1=1.59&r2=1.60)
plpgsql.h (r1.88 -> r1.89)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/plpgsql.h.diff?r1=1.88&r2=1.89)