fix issue#20: A Crash Bug in TestVisitor testcase

Interface declare will cause this bug.

```
Traceback (most recent call last):
  File "testVisitor.py", line 74, in <module>
    tree.accept(MyVisitor())
  File "/Users/sim/github/TAG_obfuscate/model.py", line 53, in accept
    type_decl.accept(visitor)
  File "/Users/sim/github/TAG_obfuscate/model.py", line 106, in accept
    decl.accept(visitor)
  File "/Users/sim/github/TAG_obfuscate/model.py", line 268, in accept
    decl.accept(visitor)
  File "/Users/sim/github/TAG_obfuscate/model.py", line 195, in accept
    for e in self.body:
TypeError: 'NoneType' object is not iterable
```