magma_free interface is broken

<div id="comment:0"></div>

Just like #10499, http://magma.maths.usyd.edu.au/calc/ is broken again from sage:

```
sage: magma_free('2+3')
Traceback (most recent call last):
...
IndexError: list index out of range
```

Component: **interfaces**

_Issue created by migration from https://trac.sagemath.org/ticket/12641_

