Crash when strings and numbers are mixed in a DAT line

This used to work, I think, but it's crashing now:

```dcpu
:data .dat "a string", 7, "more string"
```

```
Outputing to test.bin                      
Loading test.asm     
terminate called after throwing an instance of 'std::invalid_argument' 
  what():  stoi      
zsh: abort (core dumped)  dasm test.asm    
```

Splitting the same values into multiple `.dat`s works.