pgsql: Make int64_div_fast_to_numeric() more robust.

Make int64_div_fast_to_numeric() more robust.

The prior coding of int64_div_fast_to_numeric() had a number of bugs
that would cause it to fail under different circumstances, such as
with log10val2 <= 0, or log10val2 a multiple of 4, or in the "slow"
numeric path with log10val2 >= 10.

None of those could be triggered by any of our current code, which
only uses log10val2 = 3 or 6. However, they made it a hazard for any
future code that might use it. Also, since this is exported by
numeric.c, users writing their own C code might choose to use it.

Therefore fix, and back-patch to v14, where it was introduced.

Dean Rasheed, reviewed by Tom Lane.

Discussion: https://postgr.es/m/CAEZATCW8gXgW0tgPxPgHDPhVX71%2BSWFRkhnXy%2BTfGDsKLepu2g%40mail.gmail.com

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/4f74741a5cea38228fdb0fb2967fa2137ea02cbf

Modified Files
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src/backend/utils/adt/numeric.c | 80 +++++++++++++++++++++++++++--------------
1 file changed, 54 insertions(+), 26 deletions(-)