ASoC: msm: qdsp6v2: fix the crash issues due to NULL pointer

If adsp subsystem restart happens, ac and apr handles are not valid
any more. While recovering subsystem restart and reassign proper
handles protection from accessing NULL pointer is required.

CRs-fixed: 513414
Change-Id: Idb60d3504a37c0de708846a4f8517f9d75eb9078
Signed-off-by: Fred Oh <fred@codeaurora.org>