Segfault for large recursion depths

With large (~ hundreds of calls) recursion depths, syntran can crash on Linux with a segfault.  For example:

```bash
git clone https://github.com/jeffirwin/aoc-syntran
cd aoc-syntran/2018/17
syntran main.syntran
```

Outputs:
```
...
x, y = [99, 421]
x, y = [99, 422]
x, y = [99, 423]
Segmentation fault (core dumped)
```