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>.

CRs-fixed: 1009682
Change-Id: If7b3860dbe20346130a21d8687e171e8fdeb1add
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>
Git-commit: bb6b3d1230e2907273b4ec52364d157667325a12
Git-repo: https://android.googlesource.com/kernel/common/


Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>