pgsql: Fix numeric_power() when the exponent is INT_MIN.

Fix numeric_power() when the exponent is INT_MIN.

In power_var_int(), the computation of the number of significant
digits to use in the computation used log(Abs(exp)), which isn't safe
because Abs(exp) returns INT_MIN when exp is INT_MIN. Use fabs()
instead of Abs(), so that the exponent is cast to a double before the
absolute value is taken.

Back-patch to 9.6, where this was introduced (by 7d9a4737c2).

Discussion: https://postgr.es/m/CAEZATCVd6pMkz=BrZEgBKyqqJrt2xghr=fNc8+Z=5xC6cgWrWA@mail.gmail.com

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9a299dff25a5f945cc2ee0c00b705857f966e169

Modified Files
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src/backend/utils/adt/numeric.c | 2 +-
src/test/regress/expected/numeric.out | 6 ++++++
src/test/regress/sql/numeric.sql | 1 +
3 files changed, 8 insertions(+), 1 deletion(-)