pgsql: Check for overflow in strtol() while parsing datetime inputs.

Log Message:
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Check for overflow in strtol() while parsing datetime inputs.
Michael Fuhr.

Modified Files:
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pgsql/src/backend/utils/adt:
datetime.c (r1.161 -> r1.162)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/datetime.c.diff?r1=1.161&r2=1.162)