wavpack: check K, fix assertion failure

Instead of this it would be possible to change the code to support
larger K but that would make the code slower and probably still need
checks on K.
This variant is choosen as there are no known valid files that need
larger K

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
