Valgrind & tests for `numeric`

Hi

Recently I noticed, that when I'm running the regression tests under
Valgrind 3.9.0,
one test for `numeric` is constantly failing:

Here is what expected:
```
SELECT i as pow,
round((-2.5 * 10 ^ i)::numeric, -i),
round((-1.5 * 10 ^ i)::numeric, -i),
round((-0.5 * 10 ^ i)::numeric, -i),
round((0.5 * 10 ^ i)::numeric, -i),
round((1.5 * 10 ^ i)::numeric, -i),
round((2.5 * 10 ^ i)::numeric, -i)
FROM generate_series(-5,5) AS t(i);
pow | round | round | round | round | round | round
-----+----------+----------+----------+---------+---------+---------
-5 | -0.00003 | -0.00002 | -0.00001 | 0.00001 | 0.00002 | 0.00003
-4 | -0.0003 | -0.0002 | -0.0001 | 0.0001 | 0.0002 | 0.0003
-3 | -0.003 | -0.002 | -0.001 | 0.001 | 0.002 | 0.003
-2 | -0.03 | -0.02 | -0.01 | 0.01 | 0.02 | 0.03
-1 | -0.3 | -0.2 | -0.1 | 0.1 | 0.2 | 0.3
0 | -3 | -2 | -1 | 1 | 2 | 3
1 | -30 | -20 | -10 | 10 | 20 | 30
2 | -300 | -200 | -100 | 100 | 200 | 300
3 | -3000 | -2000 | -1000 | 1000 | 2000 | 3000
4 | -30000 | -20000 | -10000 | 10000 | 20000 | 30000
5 | -300000 | -200000 | -100000 | 100000 | 200000 | 300000
(11 rows)
```

and here is what I've got (see the last row):
```
SELECT i as pow,
round((-2.5 * 10 ^ i)::numeric, -i),
round((-1.5 * 10 ^ i)::numeric, -i),
round((-0.5 * 10 ^ i)::numeric, -i),
round((0.5 * 10 ^ i)::numeric, -i),
round((1.5 * 10 ^ i)::numeric, -i),
round((2.5 * 10 ^ i)::numeric, -i)
FROM generate_series(-5,5) AS t(i);
pow | round | round | round | round | round | round
-----+----------+----------+----------+---------+---------+---------
-5 | -0.00003 | -0.00002 | -0.00001 | 0.00001 | 0.00002 | 0.00003
-4 | -0.0003 | -0.0002 | -0.0001 | 0.0001 | 0.0002 | 0.0003
-3 | -0.003 | -0.002 | -0.001 | 0.001 | 0.002 | 0.003
-2 | -0.03 | -0.02 | -0.01 | 0.01 | 0.02 | 0.03
-1 | -0.3 | -0.2 | -0.1 | 0.1 | 0.2 | 0.3
0 | -3 | -2 | -1 | 1 | 2 | 3
1 | -30 | -20 | -10 | 10 | 20 | 30
2 | -300 | -200 | -100 | 100 | 200 | 300
3 | -3000 | -2000 | -1000 | 1000 | 2000 | 3000
4 | -30000 | -20000 | -10000 | 10000 | 20000 | 30000
5 | -300000 | -200000 | 0 | 0 | 200000 | 300000
(11 rows)
```

I can see this behavior on the latest master with `USE_VALGRIND` enabled. Is
it something well known?