dsp: q6lsm: Avoid null pointer access in SSR

Synchronize both contexts of SSR and SVA close
to avoid access of client handle after free.
Check whether the client is valid or not when
retrieved from private data in q6lsm_callback along
with NULL check to avoid invalidated pointer access.

Change-Id: I51c13cd79fd947c624bf0cade4c93a3fdf07353e
Signed-off-by: Aditya Bavanari <abavanar@codeaurora.org>