Fix segfault.
Fix segfault.

If the endpoint doesn't have a CLOCK_DOMAIN descriptor then the set/get clock_source commands would segfault.
audioscience/avdecc-lib,andrew-elder/avdecc-lib,andrew-elder/avdecc-lib,audioscience/avdecc-lib,andrew-elder/avdecc-lib,audioscience/avdecc-lib