[unity] Make SkeletonRenderer invalid when destroying.

spine-unity/Assets/spine-unity/Modules/SlotBlendModes/SlotBlendModes.cs

@@ -133,7 +133,7 @@ namespace Spine.Unity.Modules {
 			}
 
 			Applied = false;
-			skeletonRenderer.LateUpdate();
+			if (skeletonRenderer.valid) skeletonRenderer.LateUpdate();
 		}
 
 		public void GetTexture () {

spine-unity/Assets/spine-unity/SkeletonRenderer.cs

@@ -147,6 +147,7 @@ namespace Spine.Unity {
 
 		void OnDestroy () {
 			rendererBuffers.Dispose();
+			valid = false;
 		}
 
 		public virtual void ClearState () {