Fix walks when the prefix doesn't match any strings

Currently, the walk code doesn't check to see if any prefixes matching the
string actually exist.

--HG--
extra : amend_source : 4bedf30691bab88bf24ab8b63375ca36c3b24cc2
Fix walks for long non-matching prefixes
Merge pull request #5 from sid0/fix-empty-walk

Fix walks when the prefix doesn't match any strings