kernel/msm: Fix for target crash while playing dts content

-During dts clip playback, failure of hw params causes
substream to free.Same substream is accesed again in
asm_close which results in crash.
-Added substream NULL check in asm_close.
CRs-Fixed: 495536

Change-Id: I1cbf8b2a2b0183b0fc76cd8560f647cb803c8df5
Signed-off-by: Vivek Veenam <vveenam@codeaurora.org>
Merge "kernel/msm: Fix for target crash while playing dts content"