pgsql: Better fix for determining minimum and maximum int64 values that

Log Message:
-----------
Better fix for determining minimum and maximum int64 values that doesn't
require stdint.h and works for "busted" int64.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
xml.c (r1.30 -> r1.31)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c.diff?r1=1.30&r2=1.31)