Fix xmlTextReaderNext with preparsed document

This fixes the traversal of parent nodes using xmlTextReaderNext() when the reader is based on a preparsed document (created using xmlReaderWalker(doc)).

Without this fix the parser will abort even though there are parent nodes it should traverse to, if it is not currently on an element or attribute node. This is incorrect, since it can be for example on a text node when it needs to enter backtracking.

Fixes #6 (closed)