Panic when parsing malformed XML: "Proper bang start guaranteed by read_bang_elem"

Reproduction:

```rust
fn main() {
    let doc = "<!D>";
    let mut reader = quick_xml::Reader::from_str(doc);
    let mut buf = Vec::new();
    reader.read_event(&mut buf);
    reader.read_event(&mut buf);
}
```

Tested on 521d72a28449eaa