If `file-read-characters` reads past the end of a file, it fills in with null characters rather than just stopping

To reproduce:

1) Create some small file of like a dozen characters. Call it `test.txt` or whatever. I put in "hello world" ended with a newline.
2) Open NetLogo, do `file-open "path/to/test.txt"`
3) Do `show file-read-characters 100`.

You'll get something like:

![image](https://cloud.githubusercontent.com/assets/28215/5273849/64add47a-7a52-11e4-8ad3-b026c30eedf1.png)

(you can't really copy/paste it).

Those squares are null characters left over from the character buffer used to read in the file. It should have just returned `"hello world\n"` (in this case).