pgsql: Fix datetime input functions to correctly detect integer overflow

Log Message:
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Fix datetime input functions to correctly detect integer overflow when
running on a 64-bit platform ... strtol() will happily return 64-bit
output in that case.  Per bug #4231 from Geoff Tolley.

Tags:
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REL8_1_STABLE

Modified Files:
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    pgsql/src/backend/utils/adt:
        datetime.c (r1.160.2.5 -> r1.160.2.6)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/datetime.c?r1=1.160.2.5&r2=1.160.2.6)