avcodec/svq3: fix slice size check

Fixes out of array read
Fixes: 09f46aa2175cade93e3e3932646a56a9/asan_heap-oob_4a5385_2995_498f6abfdc0248288cefe5f4b7ad316c.mov

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