networkd: dont crash when mtu changes (#6594)

Prevent networkd from crashing when UseMTU is used. Many drivers will
bring the link down and then back up to configure a new MTU. Networkd
will also asynchonously send rtnl messages to configure the link and may
receive responses after the link has gone down and come back up (which
networkd will handle and set the lease and network to NULL.

This changes the behavior to instead return if this is the case instead
of crashing via assert.

(cherry picked from commit 0c9b15a3)