ASoC: msm: qdsp6v2: check audio client pointer before accessing

In the registered callback for q6asm, we are checking if
the audio client pointer is valid and also, dereferencing it
to get the session ID even though it could be invalid or expired.
Return and exit immediately if the audio client pointer is
invalid.

CRs-Fixed: 832914
Change-Id: I96b722b584a4b5adf8a33891abd75a320e76ea25
Signed-off-by: Aravind Kumar <akumark@codeaurora.org>