Explicit return byte array at end of function.

Very handy markdown library! However I had a problem when importing the latest version:
''src/github.com/russross/blackfriday/html.go:818: function ends without a return statement''

By explicit return a new byte array unless if hits, fixes this issue.
Edit: seems that this constraint is removed in go 1.1 (I currently run 1.0.2), however I think the fix should be included for backwards compatibility. :-)