dsp: lsm: add apr NULL check in apr deregister

During SSR, q6lsm_mmapcallback could happen before
q6lsm_mmap_apr_dereg. q6lsm_common.apr and
q6lsm_common.apr_users are cleared in q6lsm_mmapcallback.
Add apr NULL check in q6lsm_mmap_apr_dereg before accessing
q6lsm_common.apr_users.

Change-Id: Ia0ff57b3fff0a0e4f26b836a8a15324406af1a14
Signed-off-by: Meng Wang <mengw@codeaurora.org>