Initialise all 8-bytes instead of just the lower 4-bytes. This test
isn't designed to be testing whether the Seek method of the object
return by CreateStreamOnHGlobal uses the high part of the positions
passed in.

Found by Valgrind.

Found by Valgrind.

Spotted by Juan Lang.

We already store "parentDevice", so we might as well use it.

Includes checks for 64-bit file structure and avoids repeating header 
checks.

This is needed so that the new transacted type will be able to easily support
this logic.

This is to avoid accessing the top-level StorageImpl directly.

Since simple mode does not support substorages, we know these are the same.