pgsql: Improve overflow/underflow handling in regr_r2().

Improve overflow/underflow handling in regr_r2().

Commit 6498287696d improved corr()'s final function to cope with
overflow/underflow in the final calculation, and clamped its result to
[-1, 1] in case of roundoff error. Improve regr_r2() in a similar way,
clamping its result to [0, 1].

Arguably this is a bug fix, but given the lack of prior complaints,
refrain from back-patching, as we did with 6498287696d.

Reported-by: Chengpeng Yan <chengpeng_yan(at)outlook(dot)com>
Author: Chengpeng Yan <chengpeng_yan(at)outlook(dot)com>
Reviewed-by: Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/33E01656-BB3B-46E9-A41F-24A01A7C35F4@outlook.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d58ec50e0faae364fa59cdb88eba7e23b55ab583

Modified Files
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src/backend/utils/adt/float.c | 37 ++++++++++++++++++--
src/test/regress/expected/aggregates.out | 58 +++++++++++++++++++++-----------
src/test/regress/sql/aggregates.sql | 19 ++++++++---
3 files changed, 87 insertions(+), 27 deletions(-)