Mesh: Fix bug in fetching node vendor info

Mesh: Fix bug in fetching node vendor info

This patch handles following:
- Use Node's unicast address to search node in CDB.
Network UUID was wrongly used to search node in CDB,
as only local node's UUID will match with Network UUID,
rest of the nodes(provisioned) can be identified by
only their unique device UUID's. Therefore, unicast
address is the unique identifier of a node within
a network.

- NULL check was missing for node json entry before
returning not found result.

- Added few DEBUG prints.

Change-Id: I5374b21768355197e696b1ad5d12ac5d1bf143aa
Signed-off-by: Anupam Roy <anupam.r@samsung.com>