ipv6: Fix finding best source address in ipv6_dev_get_saddr().

Commit 9131f3de ("ipv6: Do not iterate over all interfaces when
finding source address on specific interface.") did not properly
update best source address available.  Plus, it introduced
possible NULL pointer dereference.

Bug was reported by Erik Kline <ek@google.com>.
Based on patch proposed by Hajime Tazaki <thehajime@gmail.com>.

Fixes: 9131f3de

 ("ipv6: Do not
	iterate over all interfaces when finding source address
	on specific interface.")
Signed-off-by: YOSHIFUJI Hideaki <hideaki.yoshifuji@miraclelinux.com>
Acked-by: Hajime Tazaki <thehajime@gmail.com>
Acked-by: Erik Kline <ek@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>