audio: ocmem: fixed 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: 592823, 589762, 592580, 590719, 589450, 593497
Change-Id: I53208de26a4d22f6b60d118e47d16a3ad29a8040
Signed-off-by: anish kumar <kanish@codeaurora.org>