ice: add NULL check in eswitch lag check

In the Linux kernel, the following vulnerability has been resolved:

ice: add NULL check in eswitch lag check

The function ice_lag_is_switchdev_running() is being called from outside of the LAG event handler code. This results in the lag->upper_netdev being NULL sometimes. To avoid a NULL-pointer dereference, there needs to be a check before it is dereferenced.