AviSplitter: Fix a crash when closing the file while re-indexing.

That crash was visible only if the user seeked while re-indexing a broken file and then aborted the re-indexing.

Fixes #2805.
Open the specified bdmv file directly if the internal mpeg splitter is disabled.

This allows a third party splitter to take full control of the title selection,
and doesn't rely on MPC-HC to make the (initial) title choice.