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: bf36123 ("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>

kernel/auditsc.c

@@ -657,7 +657,7 @@ static int audit_filter_rules(struct task_struct *tsk,
 			result = audit_comparator(audit_loginuid_set(tsk), f->op, f->val);
 			break;
 		case AUDIT_SADDR_FAM:
-			if (ctx->sockaddr)
+			if (ctx && ctx->sockaddr)
 				result = audit_comparator(ctx->sockaddr->ss_family,
 							  f->op, f->val);
 			break;