dsp: add change to fix use-after-free issue

Add change to properly handle the pointers by setting them to
NULL after free and adding some null checks before dereferencing.

Change-Id: I3e52b9a6885a8d8a91c09f75fe92ba69e3eb555f
Signed-off-by: Shashi Kant Maurya <smaury@codeaurora.org>