net: Trap attempts to call sock_kfree_s() with a NULL pointer.

Unlike normal kfree() it is never right to call sock_kfree_s() with
a NULL pointer, because sock_kfree_s() also has the side effect of
discharging the memory from the sockets quota.
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/sock.c

@@ -1718,6 +1718,8 @@ EXPORT_SYMBOL(sock_kmalloc);
  */
 void sock_kfree_s(struct sock *sk, void *mem, int size)
 {
+	if (WARN_ON_ONCE(!mem))
+		return;
 	kfree(mem);
 	atomic_sub(size, &sk->sk_omem_alloc);
 }