pgsql: Fix numeric width_bucket() to allow its first argument to be inf

Fix numeric width_bucket() to allow its first argument to be infinite.

While the calculation is not well-defined if the bounds arguments are
infinite, there is a perfectly sane outcome if the test operand is
infinite: it's just like any other value that's before the first bucket
or after the last one.  width_bucket_float8() got this right, but
I was too hasty about the case when adding infinities to numerics
(commit a57d312a7), so that width_bucket_numeric() just rejected it.
Fix that, and sync the relevant error message strings.

No back-patch needed, since infinities-in-numeric haven't shipped yet.

Discussion: https://postgr.es/m/2465409.1602170063@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8ce423b1912b8303dbec5dc3ec78a7a725acf6c2

Modified Files
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src/backend/utils/adt/numeric.c       |  5 +++--
src/test/regress/expected/numeric.out | 17 +++++++++++++----
src/test/regress/sql/numeric.sql      |  7 +++++--
3 files changed, 21 insertions(+), 8 deletions(-)