Initialize UserData property (#798)

Resolves an issue when reading slice data that has user data

Nez.Portable/Assets/Aseprite/AsepriteSlice.cs

@@ -39,6 +39,7 @@ internal AsepriteSlice(bool isNinePatch, bool hasPivot, string name)
 			HasPivot = hasPivot;
 			Name = name;
 			Keys = new List<AsepriteSliceKey>();
+            UserData = new AsepriteUserData();
 		}
 	}
-}
+}