i#1168: Fix OOB write in NT wide path conversion

buf_len should be measured in wide chars, not bytes, because we index
with it.  The out param is measured in bytes, though.

This bug was present in the initial NT wide path conversion
implementation from r1893.

Fixes issue 1168.

Review URL: https://codereview.appspot.com/7845044

git-svn-id: https://dynamorio.googlecode.com/svn/trunk@1998 49cc7528-f6fd-11dd-9d1a-b59b2e1864b6