qdsp5v2: Fix a null pointer dereference.

If ACDB data has not been filled and audio is going to be disabled, audpp_cb
accesses invalid memory address.

This scenario seems to happen when no audio is processed, but audio device is
opened and later closed.

Change-Id: I64724c7a6da8aa2c6da723a8c1122a5ae8d6796f