Fix NULL pointer dereference in cn_filter()

Check that sk_user_data is not NULL, else return from cn_filter().
Could not reproduce this issue, but Oliver Sang verified it has fixed
the "Closes" problem below.

Fixes: 2aa1f7a ("connector/cn_proc: Add filtering to fix some bugs")
Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-lkp/[email protected]/
Signed-off-by: Anjali Kulkarni <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>

Author: anjalidk

drivers/connector/cn_proc.c

@@ -54,7 +54,7 @@ static int cn_filter(struct sock *dsk, struct sk_buff *skb, void *data)
 	enum proc_cn_mcast_op mc_op;
 	uintptr_t val;
 
-	if (!dsk || !data)
+	if (!dsk || !dsk->sk_user_data || !data)
 		return 0;
 
 	ptr = (__u32 *)data;