amazing runtime error bug

```
I got a bug --- some answers got a `runtime error` result without any expalin. 
In fact, the answer is right. because it can get right output and if user 
submit more one time, oj will return ac.

our oj is on Centos 6.

```

Original issue reported on code.google.com by `zuchua...@gmail.com` on 2 Dec 2014 at 1:21
