state: nil-check waiting evals before attempting to cancel them (#26872)

When we attempt to drop unneeded evals from the eval broker, if the eval has
been GC'd before the check is made, we hit a nil pointer. Check that the eval
actually exists before attempting to remove it from the broker.

Fixes: https://github.com/hashicorp/nomad/issues/26871

.changelog/26872.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+state: Fixed a bug where the server could panic when attempting to remove unneeded evals from the eval broker
+```

nomad/fsm.go

@@ -1034,7 +1034,7 @@ func (n *nomadFSM) applyAllocClientUpdate(msgType structs.MessageType, buf []byt
 				"eval_id", evalID, "error", err)
 			return err
 		}
-		if !eval.ShouldEnqueue() {
+		if eval != nil && !eval.ShouldEnqueue() {
 			n.evalBroker.DropWaiting(eval)
 		}
 	}