audit: fix possible null-pointer dereference in audit_filter_rules

Fix  possible null-pointer dereference in audit_filter_rules.

audit_filter_rules() error: we previously assumed 'ctx' could be null

Cc: stable@vger.kernel.org
Fixes: bf361231 ("audit: add saddr_fam filter field")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>