Fixes #151 - Do not reference empty_string on webvtt_release_string

Now setting `str->d = 0;` in order to NULL the string data, rather
than unnecessarily referencing the empty string. This may help expose
poor memory management, and also help prevent the narrow chance of
overflowing the empty string refcount.
Merge pull request #151 from caitp/issue-151

Don't reference empty_string when releasing a string