Minor bug fix in Prelude.Fin.integerToFin.

Previously, the function `integerToFin` returned `Just fZ` for negative integers. It should return `Nothing`.

Before patch:
```
Idris> integerToFin (-1) 2
Just (fZ) : Maybe (Fin 2)
```

After patch:
```
Idris> integerToFin (-1) 2
Nothing : Maybe (Fin 2)
```