ASoC: msm: qdsp6v2: check if buf is null before using it

buf returned from q6asm_shared_io_buf could be null and
it could cause crash. Check if buf is null before using
it to avoid crash.

CRs-Fixed: 1046606
Change-Id: Ia988b11ff7d03edc4ddd959571da1378da62077c
Signed-off-by: Meng Wang <mwang@codeaurora.org>