pgsql: Fix overflow danger in SampleHeapTupleVisible()

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/28328ec87b45725f62bed1104d99c8b3220d1675

Modified Files
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src/backend/access/heap/heapam_handler.c | 12 +++++++++---
1 file changed, 9 insertions(+), 3 deletions(-)