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().
Reported-by: Ranier Vilela
Discussion: https://postgr.es/m/CAEudQAot_xQoZyPZjpj1aBUPrPykY5mOPHGyvfe%3Djz%2BWowdA3A%40mail.gmail.com