ipv6: fix NULL reference in proxy neighbor discovery

The addition of TLLAO option created a kernel OOPS regression
for the case where neighbor advertisement is being sent via
proxy path.  When using proxy, ipv6_get_ifaddr() returns NULL
causing the NULL dereference.

Change causing the bug was:
commit f7734fdf
Author: Octavian Purdila <opurdila@ixiacom.com>
Date:   Fri Oct 2 11:39:15 2009 +0000

    make TLLAO option for NA packets configurable

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Acked-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>