net: ipc_router: fix NULL pointer de-reference issue

Fail cases of accept() system call on AF_MSM_IPC socket family causes
NULL pointer de-reference of sock structure variable in release operation.

Validate the sock structure pointer before using it in release operation.

CRs-Fixed: 1068888
Change-Id: I5637e52be59ea9504ea6ae317394bef0c28c7865
Signed-off-by: Arun Kumar Neelakantam <aneela@codeaurora.org>