pgsql: Blind try to fix portability issue in commit 8f93bd851 et al.

Blind try to fix portability issue in commit 8f93bd851 et al.

The S/390 members of the buildfarm are showing failures indicating
that they're having trouble with the rint() calls I added yesterday.
There's no good reason for that, and I wonder if it is a compiler bug
similar to the one we worked around in d9476b838. Try to fix it using
the same method as before, namely to store the result of rint() back
into a "double" variable rather than immediately converting to int64.
(This isn't entirely waving a dead chicken, since on machines with
wider-than-double float registers, the extra store forces a width
conversion. I don't know if S/390 is like that, but it seems worth
trying.)

In passing, merge duplicate ereport() calls in float8_timestamptz().

Per buildfarm.

Branch
------
REL9_5_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/cf73c6bfc7148077cda6ff68dae551c0a2674182

Modified Files
--------------
src/backend/utils/adt/timestamp.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)