Work around lxml API abuse

Make xmlNodeDumpOutput and htmlNodeDumpFormatOutput work with corrupted
parent pointers. This used to work with the old recursive code but the
non-recursive rewrite required parent pointers to be set correctly.

Unfortunately, lxml relies on the old behavior and passes subtrees with
a corrupted structure. Fall back to a recursive function call if an
invalid parent pointer is detected.

Fixes #255.