bridge: check return value of ipv6_dev_get_saddr()

[ Upstream commit d1d81d4c3dd886d5fa25a2c4fa1e39cb89613712 ]

otherwise source IPv6 address of ICMPV6_MGM_QUERY packet
might be random junk if IPv6 is disabled on interface or
link-local address is not yet ready (DAD).

Signed-off-by: Ulrich Weber <ulrich.weber@sophos.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>