avcodec/svq3: Reintroduce slice_type

Fixes out of array read
Fixes: 1642cd3962249d6aaf0eec2836023fb6/signal_sigsegv_2557a72_2995_04efaf2ff57a052f609a3b4a2ea4e622.mov

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