BUG: withOwnerThread closes MappedMemorySegment

[Ty Young]

A bug seems to have been introduced wherein using withOwnerThread causes 
a MappedMemorySegment to close:


     File file = new File("./test");

         if(file.exists())
             file.delete();

         file.createNewFile();

         MappedMemorySegment nativeFile = 
MemorySegment.mapFromPath(file.toPath(), 0, 4096, 
FileChannel.MapMode.READ_WRITE);

         System.out.println(nativeFile.isAlive());

         MappedMemorySegment segment = 
(MappedMemorySegment)nativeFile.asSlice(4);
         segment = (MappedMemorySegment)nativeFile.asSlice(4);
         segment = 
(MappedMemorySegment)nativeFile.asSlice(4).withOwnerThread(null);

         System.out.println(nativeFile.isAlive());
         System.out.println(segment.isAlive());


which prints true, false, and then true. The original 
MappedMemorySegment was never closed, so this is unexpected.