fixed a bug at vdm compare in PrimitiveDataBuffer

src/SceneGraph/MaterialRenderList.cpp

@@ -398,7 +398,7 @@ void MaterialRenderList::ProcIndirectRender()
 
 void MaterialRenderList::Render(RenderItem *ri)
 {
-    if(*(ri->pdb)!=*last_data_buffer)        //换buf了
+    if(!last_data_buffer||*(ri->pdb)!=*last_data_buffer)        //换buf了
     {
         if(indirect_draw_count)                 //如果有间接绘制的数据，赶紧给画了
             ProcIndirectRender();

src/SceneGraph/Vulkan/VKRenderable.cpp

@@ -28,9 +28,12 @@ const int PrimitiveDataBuffer::compare(const PrimitiveDataBuffer &pdb)const
 {
     ptrdiff_t off;
 
-    off=&vdm-&pdb.vdm;
-    if(off)
-        return off;
+    if(vdm&&pdb.vdm)
+    {
+        off=(ptrdiff_t)vdm-(ptrdiff_t)pdb.vdm;
+        if(off)
+            return off;
+    }
 
     off=vab_count-pdb.vab_count;
     if(off)