ASoC: msm: qdsp6v2: validate audio client in callback

In case of single stream multiple device(SSMD) use-case audio session is
freed on first EOS. There are some chance to crash when 2nd EOS event is
reached with some delay. This make sure return properly if audio client
is not valid.

Change-Id: I3711d8e039fc37e654ca5230f3dc8784c6dba071
Signed-off-by: Fred Oh <fred@codeaurora.org>