Fix setUnfilteredPosition to escape child text nodes of rejected parents

setUnfilteredPosition was incorrectly leaving the walker's currentNode
inside a rejected parent if the target node was a TEXT_NODE. Adjusted
branching logic to ensure the walker always moves to a valid position
if the specified position is not accepted by the current filters.

Fixes #797
Merge pull request #798 from peitschie/no-step-found

Fix setUnfilteredPosition to escape child text nodes of rejected parents