Viper codegen produces incorrect code

The following code runs correctly with the default or "native" codegens, but runs and produces incorrect output with the "viper" codegen:

``` python
@micropython.viper
def fib(n:int) -> int:
    if n == 0: return 0
    if n == 1: return 1
    return fib(n - 1) + fib(n - 2)

def test():
    res = []
    for i in range(36):
        res.append(str(fib(i)))
    print(' '.join(res))

test()
```
