Fix: relayd: double unlock on viewer stream creation error

viewer_stream_create must be called with the relay stream's
lock held since 9edaf114d. A call to pthread_mutex_unlock
was forgotten in the error path of viewer_stream_create resulting
in a double-unlock in some error scenarios.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I613f67440669af52f625b7bdc4e9ae6ce75956ea