wavpack: Fix an integer overflow

Integer Overflow Checker detected an integer
overflow while FATE was running.

See: http://fate.libav.org/x86_64-linux-ioc/



Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Signed-off-by: Kostya Shishkov <kostya.shishkov@gmail.com>