small masked array performance enhancement

remove unnecessary typecast and seterr operations
With 1,000 elements, you are just measuring python call overhead.

```
In [15] N = 1000000

In [16] m = np.zeros(N,dtype=bool)

In [17] a = np.zeros(N)

In [18] b = np.zeros(N)

In [19] %timeit m.any()
10000 loops, best of 3: 54.1 µs per loop

In [20] %timeit np.copyto(a, b, casting='unsafe', where=m)
1000 loops, best of 3: 700 µs per loop
```