wined3d: Only copy the depth buffer if there is one.

dlls/wined3d/drawprim.c

@@ -2237,6 +2237,9 @@ static void depth_copy(IWineD3DDevice *iface) {
     IWineD3DDeviceImpl *This = (IWineD3DDeviceImpl *)iface;
     IWineD3DSurfaceImpl *depth_stencil = (IWineD3DSurfaceImpl *)This->depthStencilBuffer;
 
+    /* Only copy the depth buffer if there is one. */
+    if (!depth_stencil) return;
+
     /* TODO: Make this work for modes other than FBO */
     if (wined3d_settings.offscreen_rendering_mode != ORM_FBO) return;