pgsql: Fix type of iterator variable in SH_START_ITERATE

Fix type of iterator variable in SH_START_ITERATE

Also add comment to make the reasoning behind the Assert() more explicit (per
Tom).

Reported-by: Ranier Vilela
Discussion: /messages/by-id/CAEudQAocXNJ6s1VLz+hMamLAQAiewRoW17OJ6-+9GACKfj6iPQ@mail.gmail.com
Backpatch: 11-

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/727d96511ef58521bd4be0d3acb7ba88df44e762

Modified Files
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src/include/lib/simplehash.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)