pgsql: Compute max and min int8 values using unsigned arithmetic, in

Log Message:
-----------
Compute max and min int8 values using unsigned arithmetic, in hopes of
suppressing Sun Studio compiler warnings. Per Stefan.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
xml.c (r1.44 -> r1.45)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c.diff?r1=1.44&r2=1.45)