Fixes crash in the "audiofile" decoder while logging the seek error.

Checking "!open" did not work with the NFS plugin because that plugin
does not close the file automatically, unlike CURL.

This reverts commit 966c4244.

The commit was bad, because the bug was really in
NfsInputStream::DoRead(); see previous commit.

May cause assertion failure.

New base class for other InputStream implementations that run in the
I/O thread.