Fix camera position being able to become NaN with classic controls

To reproduce, hold A for about a second, then release A and press D in the same frame.

src/main/java/com/replaymod/replay/camera/ClassicCameraController.java

@@ -126,7 +126,7 @@ public class ClassicCameraController implements CameraController {
 
         lastCall = Sys.getTime();
 
-        if(direction == null || motion < THRESHOLD) {
+        if(direction == null || direction.lengthSquared() == 0 || motion < THRESHOLD) {
             return;
         }