Make results consistent when trying to seek beyond the end of sounds.

[mackron]

The various decoders in this repository are inconsistent when seeking beyond the end of the file. In some cases it will result in an errors, whereas in other cases it will clamp to the end and return success.

All decoders need to be updated so that an error is returned when trying to seek beyond the end of the file.

Original report from miniaudio: mackron/miniaudio#651