[unity] Fix clearing invalid SkeletonRenderer and SkeletonAnimation.

spine-unity/Assets/spine-unity/SkeletonAnimation.cs

@@ -129,7 +129,7 @@ namespace Spine.Unity {
 
 		protected override void ClearState () {
 			base.ClearState();
-			state.ClearTracks();
+			if (state != null) state.ClearTracks();
 		}
 
 		public override void Initialize (bool overwrite) {

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

@@ -161,14 +161,14 @@ namespace Spine.Unity {
 		}
 
 		void OnDisable () {
-			if (clearStateOnDisable)
+			if (clearStateOnDisable && valid)
 				ClearState();
 		}
 
 		protected virtual void ClearState () {
 			meshFilter.sharedMesh = null;
 			currentInstructions.Clear();
-			skeleton.SetToSetupPose();
+			if (skeleton != null) skeleton.SetToSetupPose();
 		}
 
 		public virtual void Initialize (bool overwrite) {