Avoid null-pointer dereference for PPAPI Instance BindGraphics....

webkit/plugins/ppapi/ppapi_plugin_instance.cc

Index: webkit/plugins/ppapi/ppapi_plugin_instance.cc
===================================================================
--- webkit/plugins/ppapi/ppapi_plugin_instance.cc	(revision 72376)
+++ webkit/plugins/ppapi/ppapi_plugin_instance.cc	(working copy)
@@ -458,6 +458,10 @@
     if (bound_graphics_2d()) {
       // Start the new image with the content of the old image until the plugin
       // repaints.
+      // Use ImageDataAutoMapper to ensure the image data is valid.
+      ImageDataAutoMapper mapper(bound_graphics_2d()->image_data());
+      if (!mapper.is_valid())
+        return false;
       const SkBitmap* old_backing_bitmap =
           bound_graphics_2d()->image_data()->GetMappedBitmap();
       SkRect old_size = SkRect::MakeWH(