apparmor: fix NULL sock in aa_sock_file_perm

In the Linux kernel, the following vulnerability has been resolved:

apparmor: fix NULL sock in aa_sock_file_perm

Deal with the potential that sock and sock-sk can be NULL during

socket setup or teardown. This could lead to an oops. The fix for NULL

pointer dereference in __unix_needs_revalidation shows this is at

least possible for af_unix sockets. While the fix for af_unix sockets

applies for newer mediation this is still the fall back path for older

af_unix mediation and other sockets, so ensure it is covered.