Fix fields getting wrong parent

Hi!

There is a problem with nested messages when any sibling element following message declaration get this message as a parent. It can be reproduced in this case:
```
message Foo
{
    message Bar
    {
    }
    optional Bar bar = 1;
}
```
which results in
```
Foo->_fields = array()
Foo->_nested[Bar]->fields = array(bar)
```
instead of expected
```
Foo->_fields = array(bar)
Foo->_nested[Bar]->fields = array()
```

Here comes the patch ;-)