bridge: fix crash when set mac address of br interface

When I tried to set mac address of a bridge interface to a mac
address which already learned on this bridge, I got system hang.

The cause is straight forward: function br_fdb_change_mac_address
calls fdb_insert with NULL source nbp. Then an fdb lookup is
performed. If an fdb entry is found and it's local, it's OK. But
if it's not local, source is dereferenced for printk without NULL
check.

Signed-off-by: Hong Zhiguo <honkiko@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>