MULTICAST_IF, &addr, 1);
	printf("errno is %d\n", errno);

Xiaotian Feng(dfeng@redhat.com) caught the bug. We fix it firstly checking
the availability of optlen and then dealing with the logic like other options.

Reported-by: Xiaotian Feng <dfeng@redhat.com>
Signed-off-by: Shan Wei <shanwei@cn.fujitsu.com>
Acked-by: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>
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