bad nested lists are not interpreted usable for cleanup

with lists generated from a contenteditable in a webkit environment (qt 4.8) a implicit correction occurs: subsequent lists are rendered as sub-items of the previous list-item as if they were defined in correct order.
the w3c does not recommend or allow lists that contain lists without a li-tag arround them:
http://www.w3.org/TR/html/grouping-content.html#the-ul-element
webkit behavior see: http://jsfiddle.net/9d6TT/

feeding gumbo such a list does not result in the correct tag-hierarchy

the expected behavior of gumbo is to "correct" the invalid nesting of the lists and  return a correct tag-hierarchy

content fed to gumbo:
```
<ul><li>item1</li><ul><li>item1.1</li></ul><li>item2</li></ul>
```

tag hierarchy expected from gumbo:
```
<ul><li>item1<ul><li>item1.1</li></ul></li><li>item2</li></ul>
```

tag hierarchy got from gumbo:
```
<ul><li>item1</li><ul><li>item1.1</li></ul><li>item2</li></ul>
```

reproduce: 
make check
gdb --args ./lib/gumbo_test
break parser.cc:1660