pgsql: Cosmetic fixes for radix sort

Cosmetic fixes for radix sort

Do minor comment fixes and remove implicit cast to Datum.

While here, let's prefer crashing instead of entering an infinite
loop in case of future programming mistakes when computing next_level,
suggested by ChangAo Chen.

Discussion: https://postgr.es/m/tencent_49E3F11E74D8A584A2144ED532A490CBC40A@qq.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ca9807dfec5b70751c3524de264e4a16afb2a94a

Modified Files
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src/backend/utils/sort/tuplesort.c | 11 +++++++----
1 file changed, 7 insertions(+), 4 deletions(-)