byteOffset and byteCount are -1 when a file is closed

I just checked in the fix for a bug which provides an additional "feature": Once an instance of openFile is closed, its byteOffset and byteCount properties will be -1 rather than cause an exception if you try to access them. This provides a way to test whether a given openFile is open or closed since -1 is invalid in all other cases. Thanks to a chat room whose name I may not speak aloud for enduring my confused blunderings on this.