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>
int ping_init_sock(struct sock *sk)
{
	struct net *net = sock_net(sk);
	kgid_t group = current_egid();
	struct group_info *group_info = get_current_groups();
	int i, j, count = group_info->ngroups;
	kgid_t low, high;

	inet_get_ping_group_range_net(net, &low, &high);
	if (gid_lte(low, group) && gid_lte(group, high))
		return 0;

	for (i = 0; i < group_info->nblocks; i++) {
		int cp_count = min_t(int, NGROUPS_PER_BLOCK, count);
		for (j = 0; j < cp_count; j++) {
			kgid_t gid = group_info->blocks[i][j];
			if (gid_lte(low, gid) && gid_lte(gid, high))
				return 0;
		}

		count -= cp_count;
	}

	return -EACCES;
}

int ping_init_sock(struct sock *sk)
{
	struct net *net = sock_net(sk);
	kgid_t group = current_egid();
	struct group_info *group_info = get_current_groups();
	int i, j, count = group_info->ngroups;
	kgid_t low, high;

	inet_get_ping_group_range_net(net, &low, &high);
	if (gid_lte(low, group) && gid_lte(group, high))
		return 0;

	for (i = 0; i < group_info->nblocks; i++) {
		int cp_count = min_t(int, NGROUPS_PER_BLOCK, count);
		for (j = 0; j < cp_count; j++) {
			kgid_t gid = group_info->blocks[i][j];
			if (gid_lte(low, gid) && gid_lte(gid, high))
				return 0;
		}

		count -= cp_count;
	}

	return -EACCES;
}
