Otherwise there is no way to detect an error returned by avio_close() because
ff_format_io_close cannot get the return value.

Checking the return value of the close function is important in order to check
if all data was successfully written and the underlying close() operation was
successful.

It can also be useful even for read mode because it can return any pending
AVIOContext error, so the user don't have to manually check AVIOContext->error.

In order to still support if the user overrides io_close, the generic code only
uses io_close2 if io_close is either NULL or the default io_close callback.
Signed-off-by: Marton Balint <cus@passwd.hu>