[upstream] Make dynamic tree more serialization friendly (erincatto/box2d#1036)

minor change

Author: ikpil

src/Box2D.NET/B2DynamicTrees.cs

@@ -56,6 +56,11 @@ internal static ushort b2MaxUInt16(ushort a, ushort b)
         public static B2DynamicTree b2DynamicTree_Create()
         {
             B2DynamicTree tree = new B2DynamicTree();
+            tree.Clear();
+            
+            // memset needed for deterministic serialization
+            // memset( &tree, 0, sizeof( b2DynamicTree ) );
+
             tree.root = B2_NULL_INDEX;
 
             tree.nodeCapacity = 16;