Unsafe publication of java.util.Properties.map

JDK-8029891 refactored java.util.Properties to use a transient ConcurrentHashMap that is read from without synchronization. However, I think this implementation opens up the possibility of the map being observed as null in certain methods (such as get) when called from other threads.

One way to fix this would be by making the map field final (which would necessitate using something like Unsafe.putObjectVolatile to set the field during deserialization).