Fixes the panic mentioned in
https://github.com/hashicorp/nomad/issues/3420#issuecomment-341666932

While a leader task dying serially stops all follower tasks, the
synchronizing of state is asynchrnous. Nomad can shutdown before all
follower tasks have updated their state to dead thus saving the state
necessary to hit this panic: *have a non-terminal alloc with a dead
leader.*

The actual fix is a simple nil check to not assume non-terminal allocs
leader's have a TaskRunner.