soc: qcom: ipc_router_smd_xprt: Set pointer to NULL after free

in_pkt pointer is holding dangling pointer address even after calling
release_pkt() which causing use-after-free.

Set the in_pkt pointer to NULL after free.

CRs-Fixed: 2210859
Change-Id: If5e01c0109c947e52f3ff269c9b2b50ac0dc2bdf
Signed-off-by: Arun Kumar Neelakantam <aneela@codeaurora.org>