String conversion problem fix

[kevindehecker]

There is a in the size, e.g. 1281, which could not be directly converted to an int

[flixr]

I think a more generic solution would be something like

def eval_int(expr):
    import ast
    return int(eval(compile(ast.parse(expr, mode='eval'), '<string>', 'eval')))

and then using this: offset += eval_int(f.length)

[kevindehecker]

Yes, it is. Done