This patch fixes a NULL pointer exception which occurs when a
substream is opened and immediately closed.

Signed-off-by: Frank Mandarino <fmandarino@endrelia.com>
Signed-off-by: Liam Girdwood <lg@opensource.wolfsonmicro.com>

-- 
Frank Mandarino                                fmandarino(a)endrelia.com
Endrelia Technologies Inc.
Toronto, Ontario, Canada