fib_frontend: fix possible NULL pointer dereference

fib_frontend: fix possible NULL pointer dereference

The two commits 2c4dc291fa (net: remove delay at device dismantle) and
b0c02f7d599 (net: reinstate rtnl in call_netdevice_notifiers()) silently
removed a NULL pointer check for in_dev since Linux 3.7.

This patch re-introduces this check as it causes crashing the kernel when
setting small mtu values on non-ip capable netdevices.

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>