diag: Add protection while accessing socket info's handle

Currently, there is a possibility of passing invalid NULL handle
to selinux function after closing the channel. The patch adds
protection while accessing socket info's handle.

CRs-Fixed: 2111893
Change-Id: I94515cc52352446d61a710ed1c09818a78c39f46
Signed-off-by: Hardik Arya <harya@codeaurora.org>