Don't try to print negative ascii values

Steps to reproduce:

```
calculon v1.0

   h  0x0
   d  0
   o  0
   a  ........
   u

  63  0 0 0 0  0 0 0 0    0 0 0 0  0 0 0 0  48
  47  0 0 0 0  0 0 0 0    0 0 0 0  0 0 0 0  32
  31  0 0 0 0  0 0 0 0    0 0 0 0  0 0 0 0  16
  15  0 0 0 0  0 0 0 0    0 0 0 0  0 0 0 0  0

>>> -1
```

Press return. Lol.