return nullptr after accessing objects in other domain was denied

Fix rogerwang/node-webkit#1573

Conflicts:
	Source/bindings/v8/ScriptValue.cpp
Prevent substituteData removal from memory cache before parsing it

This issue happens sometimes during SVG image loading
Fix rogerwang/node-webkit#2351

Conflicts:
	Source/core/fetch/ResourceFetcher.cpp
	Source/core/fetch/ResourceFetcher.h
more guarding of handle scope