Fixed crash when parsing hex escape sequences

Fixed a typo which resulted in a crash when parsing hexadecimal escape sequences. 

Before:

```
$ shine
Shine 0.0.2 -- Copyright (C) 2013-2014 Richard Hundt.
shine> print("\x3a")
bad argument #1 to '?' (number expected, got nil)
```

After:

```
$ shine
Shine 0.0.2 -- Copyright (C) 2013-2014 Richard Hundt.
shine> print("\x3a")
:
```