dsp: add null check for temp handle

Variable mem_state in audio client(ac) is set to zero
when there is a successful memory mapping. However the
same variable is updated for various mapping commands.
Ensure to check for both memstate and specific mem_handle
to be updated in the wait condition to wake up the right
waiting command.

Change-Id: Iabba61cef0a90f636de50e9d27eecf886dc59a27
Signed-off-by: Meng Wang <mwang@codeaurora.org>