fix unused variable `offset` in file.c
fix unused variable `offset` in file.c

```
file.c:411:6: warning: unused variable 'offset' [-Wunused-variable]
        int offset;
            ^
file.c:438:6: warning: unused variable 'offset' [-Wunused-variable]
        int offset;
            ^
```

gabordemooij/citrine,takano32/citrine,gabordemooij/citrine,takano32/citrine,gabordemooij/citrine,takano32/citrine