a stream could be seekable in the range the ReadState already buffered
bytes, even we mark it's unseekable for the jump seek, but there still be
chance the ReadState already buffered the target position.

you can just return -1 here when m_seekable is false, but not in release
phase, we need let the time test the code. keep max existed code function,
just fixing bug, is what we can only do in this phase.