ASoC: msm: qdsp6v2: add check for NULL dereference

The asm loopback driver during close does not check
for NULL before closing the playback and capture
instances.

The change ensures only the valid playback/capture
instances are closed.

CRs-Fixed: 643724

Change-Id: I698d8fb75ce838635ed6dd70eedb6b509b9fcec6
Signed-off-by: Aravind Kumar <akumark@codeaurora.org>
Signed-off-by: Pradosh Das <c_prados@qti.qualcomm.com>