net: context: Fix the multicast address lookup in bind

Because of the change in next commit "net: if: Change
the iface param in net_if_ipv6_maddr_lookup",
we must initialize the network interface to NULL.
Without this the multicast address lookup will fail.

Change-Id: I113b44ce23c5f2ecbbf1698972078f102995e891
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>