elliptic12 sometimes gives incorrect results

The function `elliptic12` returns incorrect results for phase equal to some odd multiples of `pi/2`.  For example

```matlab
m = 0.5;
n = 17;
elliptic12(n*pi/2, m) - n*ellipke(m)
==>
ans = -1.8541
```

The correct answer is `ans = 0` (for any integer `n`).
