Thread: pgsql: Prevent corr() from returning the wrong results for negative

Log Message:
-----------
Prevent corr() from returning the wrong results for negative correlation
values. The previous coding essentially assumed that x = sqrt(x*x), which
does not hold for x < 0.

Thanks to Jie Zhang at Greenplum and Gavin Sherry for reporting this
issue.

Modified Files:
--------------
    pgsql/src/backend/utils/adt:
        float.c (r1.150 -> r1.151)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/float.c?r1=1.150&r2=1.151)