lxml.html.clean.Cleaner crushes on some HTMLs

[Nikita Vostretsov]

When I run clean_reproduce.py script exception is raised

python ~/tmp/clean_reproduce.py
Executing python /home/nikita/tmp/clean_reproduce.py in /home/nikita/ves/lxml-4.4.0/
Python : sys.version_info(major=3, minor=6, micro=7, releaselevel='final', serial=0)
lxml.etree : (4, 4, 0, 0)
libxml used : (2, 9, 9)
libxml compiled : (2, 9, 9)
libxslt used : (1, 1, 33)
libxslt compiled : (1, 1, 33)
Traceback (most recent call last):
  File "/home/nikita/tmp/clean_reproduce.py", line 30, in <module>
    main()
  File "/home/nikita/tmp/clean_reproduce.py", line 26, in main
    cleaner.clean_html(node)
  File "src/lxml/html/clean.py", line 520, in lxml.html.clean.Cleaner.clean_html
  File "src/lxml/html/clean.py", line 394, in lxml.html.clean.Cleaner.__call__
  File "/home/nikita/ves/lxml-4.4.0/lib/python3.6/site-packages/lxml/html/__init__.py", line 339, in drop_tree
    assert parent is not None

[scoder (scoder)]

I agree that it shouldn't run into that assertion. PR welcome.