Prevent sorting from requesting a SortTuple array that exceeds MaxAllocSize;

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 ...