pgsql: Fix numeric_mul() overflow due to too many digits after decimal

Fix numeric_mul() overflow due to too many digits after decimal point.

This fixes an overflow error when using the numeric * operator if the
result has more than 16383 digits after the decimal point by rounding
the result. Overflow errors should only occur if the result has too
many digits *before* the decimal point.

Discussion: https://postgr.es/m/CAEZATCUmeFWCrq2dNzZpRj5+6LfN85jYiDoqm+ucSXhb9U2TbA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e7fc488ad67caaad33f6d5177081884495cb81cb

Modified Files
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src/backend/utils/adt/numeric.c       | 10 +++++++++-
src/test/regress/expected/numeric.out |  6 ++++++
src/test/regress/sql/numeric.sql      |  2 ++
3 files changed, 17 insertions(+), 1 deletion(-)