add a null check to JsTTDNotifyContextDestroy
add a null check to JsTTDNotifyContextDestroy

JsTTDNotifyContextDestroy may be called during shutdown / destroying
threadContext. ThreadContext::GetContextForCurrentThread() may return
nullptr.

Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore