Incorrect results from corr()

The builtin corr() aggregate doesn't produce the correct results in some
circumstances. Per the SQL spec, corr(x, y) is defined as equivalent to
covar_pop(x, y) / (stddev_pop(x) * stddev_pop(y)).

postgres=# create table t1 (x float8, y float8);
CREATE TABLE
postgres=# copy t1 from stdin with csv;
0.940839937888086,0.539830380585045
0.84795232815668,0.396385048050433
0.601479615084827,0.899995123967528
0.785623408854008,0.302559469360858
0.829138438683003,0.0211085784249008
0.926528611686081,0.315794581547379
0.25934984581545,0.609216409735382
0.976522764191031,0.877208305988461
\.
postgres=# select corr(x, y) from t1;
       corr
-------------------
 0.214150892978763
(1 row)

postgres=# select covar_pop(x, y) / (stddev_pop(x) * stddev_pop(y)) from
t1;
      ?column?
--------------------
 -0.214150892978763
(1 row)

With the attached patch, we get the expected results:

postgres=# select corr(x, y) from t1;
        corr
--------------------
 -0.214150892978763
(1 row)

Credit: Jie Zhang at Greenplum and Gavin Sherry for reporting the issue.

Barring any objections, I'll apply this to HEAD and 8.2 later tonight or
tomorrow.

-Neil