msm: msm_bus: Disallow single node paths

Reject clients that attempt to register paths that have the
same source and destination. Add additional null ptr checks for
bus nodes that have no fabrice devices attached to them.

Change-Id: I13320371b2b19f6047853ffc7ffe529c0bfbd620
Signed-off-by: David Dai <daidavid1@codeaurora.org>