netfilter: bridge-netfilter: fix crash in br_nf_forward_finish()

[ 4593.956206] BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
[ 4593.956219] IP: [<ffffffffa03357a4>] br_nf_forward_finish+0x154/0x170 [bridge]
[ 4593.956232] PGD 195ece067 PUD 1ba005067 PMD 0
[ 4593.956241] Oops: 0000 [#1] SMP
[ 4593.956248] last sysfs file:
/sys/devices/LNXSYSTM:00/LNXSYBUS:00/PNP0A08:00/device:08/ATK0110:00/hwmon/hwmon0/temp2_label
[ 4593.956253] CPU 3
...
[ 4593.956380] Pid: 29512, comm: kvm Not tainted 2.6.34-rc7-net #195 P6T DELUXE/System Product Name
[ 4593.956384] RIP: 0010:[<ffffffffa03357a4>]  [<ffffffffa03357a4>] br_nf_forward_finish+0x154/0x170 [bridge]
[ 4593.956395] RSP: 0018:ffff880001e63b78  EFLAGS: 00010246
[ 4593.956399] RAX: 0000000000000608 RBX: ffff880057181700 RCX: ffff8801b813d000
[ 4593.956402] RDX: 0000000000000000 RSI: 0000000000000002 RDI: ffff880057181700
[ 4593.956406] RBP: ffff880001e63ba8 R08: ffff8801b9d97000 R09: ffffffffa0335650
[ 4593.956410] R10: 0000000000000000...