The 'fdb' is the AP's forwarding DB table. When an AP receives a packet, it will try to add the MAC into the table. It seems a packet might have got corrupted, "fdb->dst is null", but this is a warning msg, and not impacting the AP function.
Do you see these messages frequently, and/or are there any type of client problems associated?
Did this ever get solved? I have a ruckus r600 Unleashed that's doing the exact same thing.
In my case, this corrupt packet bubbles up to the DHCP server has a new request, which issues a new IP to the AP. After a while, the AP gets confused (since it is holding onto the old IP, but now has a new IP from the DHCP server).
... and to resolve this weirdness, the AP has to reboot itself thereby losing network.