relocate_written_data is called in the following situation:
* track headers need to be re-written
* at least one frame has been written already
* the space left right after the track headers does not suffice to
expand the track headers
In such a case all frames that have been written already will be
moved.
However, in certain split modes then current file may actually be a
null I/O, meaning that the current output is discarded. A null I/O
object doesn't return anything when reading for it, causing an endless
loop in the relocation code which calls `read` as often as needed
until everything's been read — which can never happen with a null I/O
object.
However, it makes no sense to try to actually read the data in such a
case, as it will be discarded anyway. Therefore just avoid trying to
read the data in the first place.
Fixes#1944.
