audio: ocmem: fix the ocmem driver crash during video playback

Triggering SSR during wait for client callback by the audio
ocmem driver causes de-reference of freed memory in ocmem_free
function. Fixed by assigning NULL to the ocmem buffer pointer after every
ocmem_free function call.

CRs-Fixed: 589762
Signed-off-by: Anish Kumar <kanish@codeaurora.org>
Change-Id: I53208de26a4d22f6b60d118e47d16a3ad29a8040