Fix ydn-db#90: ada1971 only fix reverseTraverse.
Fix ydn-db#90: ada1971 only fix reverseTraverse.

ada1971 was a misfire, sorry, this commit add call to func in traverse
method and use null parameter instead of undefined. Using null seems to
be more consistent with the last line of traverse and reverseTraverse
methods.

yathit/ydn-base,yathit/ydn-base,yathit/ydn-base,yathit/ydn-base