ra144enc: fix two "may be used uninitialized in this function" warnings

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
wavpack: check pointer to avoid overreading input buffer

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