Fix osync_obj_engine_nth_sinkengine() interface which could return on a mixed-syncing group NULL even if _num_sinkengine() reports a different limit.
Fix osync_obj_engine_nth_sinkengine() interface which could return on a
mixed-syncing group NULL even if _num_sinkengine() reports a different
limit.

Spotted by bricks.



git-svn-id: e31799a7ad59d6ea355ca047c69e0aee8a59fcd3@5402 53f5c7ee-bee3-0310-bbc5-ea0e15fffd5e

luizluca/opensync-luizluca,luizluca/opensync-luizluca