avcodec/ass_split: Change order of operations in ass_split_section()

This matches the other branch
Fixes out of array read
Fixes: 4d142ca76d39fe685effcf5017098723/asan_heap-oob_31ae824_8611_348fdb64f9009b63c8a8eae9a0e497c5.mkv

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
