pgsql: Fix datetime input functions to correctly detect integer overflow

Log Message:
-----------
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.

Modified Files:
--------------
    pgsql/src/backend/utils/adt:
        datetime.c (r1.189 -> r1.190)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/datetime.c?r1=1.189&r2=1.190)