Fix silly oversight in patch to remove dsm state file.

I'm not sure if this is what's causing the Windows buildfarm members
to get unhappy, but I don't think it can be helping anything...

src/backend/port/win32_shmem.c

@@ -253,7 +253,7 @@ PGSharedMemoryCreate(Size size, bool makePrivate, int port,
 	UsedShmemSegSize = size;
 	UsedShmemSegID = hmap2;
 
-	*shim = NULL;
+	*shim = hdr;
 	return hdr;
 }