ping: prevent NULL pointer dereference on write to msg_name

A plain read() on a socket does set msg->msg_name to NULL. So check for
NULL pointer first.

Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
void *ping_seq_next(struct seq_file *seq, void *v, loff_t *pos)
{
	struct sock *sk;

	if (v == SEQ_START_TOKEN)
		sk = ping_get_idx(seq, 0);
	else
		sk = ping_get_next(seq, v);

	++*pos;
	return sk;
}

void *ping_seq_next(struct seq_file *seq, void *v, loff_t *pos)
{
	struct sock *sk;

	if (v == SEQ_START_TOKEN)
		sk = ping_get_idx(seq, 0);
	else
		sk = ping_get_next(seq, v);

	++*pos;
	return sk;
}
