sockstat: Fix segfault when peer has no pid

[dan.mcgregor_usask.ca]

Summary

If a dgram socket's remote end is the kernel, there may not
be a file associated with it. I've seen this with gssd's kernel
rpc socket. Arguably the kernel should do something better, but
for now handle the possibility.

Signed-off-by: Dan McGregor <[e-mail]>

[dan.mcgregor_usask.ca]

Thanks, @tuexen and @emaste. This does affect my NFS server on every version I've tried in the 14-STABLE or 15-CURRENT series. 13.3 is unaffected. Looks like everything since commit a83d596f434f in 2022.

I have no commit bit, so if somebody could commit this it'd be appreciated.