Aggregate Function corr does not always return the correct value

Hi!

One of the clients complained as to why the query for calculating the
correlation coefficient with the CORR function yielded such weird
results. After a little analysis, it was discovered that they were
calculating the correlation coefficient for two sets, one of which is
more or less random and the other of which is simply a set of constant
values (0.09 if that matters). As a result, they were attaining
unexpected results. However, as far as I am aware, they should have
received NULL because it is impossible to calculate the standard
deviation for such a set.

It turns out that for some values, the function does not return NULL.
Here is a reproducing of the problem:

==============================
postgres=# WITH dataset AS (SELECT x, 0.125 AS y FROM generate_series(0, 5)
AS x) SELECT corr(x, y) FROM dataset;
corr
------

(1 row)

postgres=# WITH dataset AS (SELECT x, 0.1 AS y FROM generate_series(0, 5)
AS x) SELECT corr(x, y) FROM dataset;
corr
--------------------
0.6546536707079771
(1 row)

==============================

I'm not sure how to fix that yet, but the issue is that the Sxx and Syy
variables in the float8_corr function are very close to 0, but not zero,
so we can't return NULL.

--
Best regards,
Maxim Orlov.