pgsql: Fix behavior of float aggregates for single Inf or NaN inputs.

Fix behavior of float aggregates for single Inf or NaN inputs.

When there is just one non-null input value, and it is infinity or NaN,
aggregates such as stddev_pop and covar_pop should produce a NaN
result, because the calculation is not well-defined.  They used to do
so, but since we adopted Youngs-Cramer aggregation in commit e954a727f,
they produced zero instead.  That's an oversight, so fix it.  Add tests
exercising these edge cases.

Affected aggregates are

 var_pop(double precision)
 stddev_pop(double precision)
 var_pop(real)
 stddev_pop(real)
 regr_sxx(double precision,double precision)
 regr_syy(double precision,double precision)
 regr_sxy(double precision,double precision)
 regr_r2(double precision,double precision)
 regr_slope(double precision,double precision)
 regr_intercept(double precision,double precision)
 covar_pop(double precision,double precision)
 corr(double precision,double precision)

Back-patch to v12 where the behavior change was accidentally introduced.

Report and patch by me; thanks to Dean Rasheed for review.

Discussion: https://postgr.es/m/353062.1591898766@sss.pgh.pa.us

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/874372a941a6ee31ba4034db0367e0929e5f49bc

Modified Files
--------------
src/backend/utils/adt/float.c            |  35 +++++++++++
src/test/regress/expected/aggregates.out | 105 ++++++++++++++++++++++++++++++-
src/test/regress/sql/aggregates.sql      |  21 ++++++-
3 files changed, 159 insertions(+), 2 deletions(-)