Fix overflow danger in SampleHeapTupleVisible(), take 2
28328ec87b45725 addressed one overflow danger in
SampleHeapTupleVisible() but introduced another, albeit a less likely
one. Modify the binary search code to remove this danger.
Reported-by: Richard Guo
Reviewed-by: Richard Guo, Ranier Vilela
Fix overflow danger in SampleHeapTupleVisible()
68d9662be1c4b70 made HeapScanDesc->rs_ntuples unsigned but neglected to
change how it was being used in SampleHeapTupleVisible().
Return early if rs_ntuples is 0 to avoid overflowing and incorrectly
executing the loop code in SampleHeapTupleVisible(
