Fix crashes caused by _next_pos and cast to string

Given the following (a syntax error positioned at the end() of the iterator):
```xml
<root name="foo" size="10">
  <child name="child1"/>
  <child name="child2"/>
</root <-- syntax error!
```

A syntax error in this case implements _next_post. _next_pos increments input::iterator m_it, increments the iterator past the end(), resulting in undefined behavior and crashing.

Nice library, I enjoyed working with it. It's a much more natural approach than most C++ parsing libraries. :clap: