Add  memset to clear buffer to unparse uuid into.

- jack_remove_properties and jack_get_properties both search the metadata database for a matching UUID string.
- However, they both fail at finding any matching UUIDs ever.
- The UUID string in the database are correctly created on a previously cleared buffer (e.g. memset to 0).
- The temporaty UUID string in the search routines however are not created on a cleared buffer, the end of the buffer thus may contain anything.
- As in the matching routine the complete UUID strings are compared (with size JACK_UUID_STRING_SIZE), there will never be a matching pair, even if their valid part do match.

Fixes Issue #8