SoC: msm: q6dspv2: add null check for payload

In debug message, payload could be NULL and cause panic. Remove
payload from debug message.
Add null check for payload to avoid panic.

CRs-Fixed: 1087543
Change-Id: I3e2fffdddfb7822a4eb3e142738363f1e80522d8
Signed-off-by: Meng Wang <mwang@codeaurora.org>