pgsql: Prevent sorting from requesting a SortTuple array that exceeds

Log Message:
-----------
Prevent sorting from requesting a SortTuple array that exceeds MaxAllocSize;
we'll go over to disk-based sort if we reach that limit.
This fixes Stefan Kaltenbrunner's observation that sorting can suffer an
'invalid memory alloc request size' failure when sort_mem is set large
enough.  It's unfortunately not so easy to fix in 8.1 ...

Modified Files:
--------------
    pgsql/src/backend/utils/sort:
        tuplesort.c (r1.60 -> r1.61)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/sort/tuplesort.c.diff?r1=1.60&r2=1.61)