[IPV6]: Check outgoing interface even if source address is unspecified.

The outgoing interface index (ipi6_ifindex) in IPV6_PKTINFO
ancillary data, is not checked if the source address (ipi6_addr)
is unspecified.  If the ipi6_ifindex is the not-exist interface,
it should be fail.

Based on patch from Shan Wei <shanwei@cn.fujitsu.com> and
Brian Haley <brian.haley@hp.com>.

Signed-off-by: Shan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: Brian Haley <brian.haley@hp.com>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>