pgsql: Make the system-attributes loop in AddNewAttributeTuples depend

Log Message:
-----------
Make the system-attributes loop in AddNewAttributeTuples depend on
lengthof(SysAtt) not FirstLowInvalidHeapAttributeNumber, for consistency with
the other uses of the SysAtt array, and to make it clearer that it doesn't
walk off the end of that array.

Modified Files:
--------------
    pgsql/src/backend/catalog:
        heap.c (r1.340 -> r1.341)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/heap.c?r1=1.340&r2=1.341)