Fix {NaN, NaN} slider paths differently

Previous fix simply accepts that startPoint can equal endPoint, which shouldn't ever be the case.

If two nodes are stacked, they would have no distance between them, and as such not form a path at all.

This fixes that too.

objects/hitobjects/Slider.cs

@@ -491,12 +491,12 @@ private Vector2 GetLinearPathPosition(double time)
             int prevNodeIndex = 0;
             foreach(double pathLength in pathLengths)
             {
+                ++prevNodeIndex;
+
                 if (fractionDistance > pathLength)
                     fractionDistance -= pathLength;
                 else
                     break;
-
-                ++prevNodeIndex;
             }
 
             if (prevNodeIndex >= nodePositions.Count())
@@ -507,11 +507,6 @@ private Vector2 GetLinearPathPosition(double time)
             double pointDistance  = GetDistance(startPoint, endPoint);
             double microFraction  = fractionDistance / pointDistance;
 
-            if (pointDistance == 0)
-                // With no distance between points, we will simply end up with the start point.
-                // Note that `microFraction` is NaN in this case, so without this we would return {NaN, NaN}.
-                return startPoint;
-
             return startPoint + new Vector2(
                 (endPoint - startPoint).X * (float)microFraction,
                 (endPoint - startPoint).Y * (float)microFraction