Fix segfaults if comment is present in sequence 

If sequence element contains comment node right after the length element, 
```
first_elem = first_elem->NextSibling()->ToElement();
```
returns NULL, so 
```
if (first_elem->NextSibling() == 0)
```
dereferences zero pointer, thus making fast_type_gen crash (I've got this behaviour on Moscow Exchange fast templates)

XMLElement::NextSiblingElement() skips all non-element items, fixing this error.