Fixes use of uninitialized data, as alternative alpha could be
calculated conditionally

Fixes part of msan_uninit-mem_7f51a8b0b3b0_1009_Arlington.wmv
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>