pgsql: Fix potential integer handling issue in radixtree.h.

Fix potential integer handling issue in radixtree.h.

Coverity complained about the integer handling issue; if we start with
an arbitrary non-negative shift value, the loop may decrement it down
to something less than zero before exiting. This commit adds an
assertion to make sure the 'shift' is always 0 after the loop, and
uses 0 as the shift to get the key chunk in the following operation.

Introduced by ee1b30f12.

Reported-by: Tom Lane as per coverity
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/2089517.1711299216%40sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/80d5d4937c168af46c976feebe24765ad76eb540

Modified Files
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src/include/lib/radixtree.h | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)