pgsql: Make rs_cindex and rs_ntuples unsigned

Make rs_cindex and rs_ntuples unsigned

HeapScanDescData.rs_cindex and rs_ntuples can't be less than 0. All scan
types using the heap scan descriptor expect these values to be >= 0.
Make that expectation clear by making rs_cindex and rs_ntuples unsigned.

Also remove the test in heapam_scan_bitmap_next_tuple() that checks if
rs_cindex < 0. This was never true, but now that rs_cindex is unsigned,
it makes even less sense.

While we are at it, initialize both rs_cindex and rs_ntuples to 0 in
initscan().

Author: Melanie Plageman
Reviewed-by: Dilip Kumar
Discussion: https://postgr.es/m/CAAKRu_ZxF8cDCM_BFi_L-t%3DRjdCZYP1usd1Gd45mjHfZxm0nZw%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/68d9662be1c4b705123a0e292974fb4be661294c

Modified Files
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src/backend/access/heap/heapam.c | 7 +++++--
src/backend/access/heap/heapam_handler.c | 2 +-
src/include/access/heapam.h | 4 ++--
3 files changed, 8 insertions(+), 5 deletions(-)