kern/83999 : [firewire] [patch] panic in fw_bus_explore due to NULL reference

[Cai, Quanqing]

Can somebody please have a look at this and apply the patch? However,
I can't re-produce this bug, but the fix does not hurt anything. I
made the changement on my two machines according the patch, and
everything looks and works good. One of machine is 7.0-CURRENT,
another one is 6-STABLE.

BR
Cai, Quanqing