FROMLIST: msm: qdsp6v2: set pointer to NULL after free

Pointer after kfree is not sanitized.
Set pointer to NULL.

Conflicts:
	sound/soc/msm/qdsp6v2/msm-compress-q6-v2.c
	sound/soc/msm/qdsp6v2/msm-pcm-q6-noirq.c

Bug: 38177011
Bug: 37275839
CRs-Fixed: 2008031
Change-Id: Ia59a57fcd142a6ed18d168992b8da4019314afa4
Signed-off-by: Xiaojun Sang <xsang@codeaurora.org>
Signed-off-by: Siqi Lin <siqilin@google.com>
(am from https://source.codeaurora.org/quic/la/kernel/msm-3.18/commit/?id=30baaec8afb05abf9f794c631ad944838d498ab8)