avcodec/fic: fix slice checks

fix integer overflows

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
avcodec/fic: clear slice_data

Fixes artifacts
Fixes use of freed memory

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
avcodec/fic: Fix return value check

Fixes part of Ticket3466
Found-by: Andrey_Karpov / PVS-Studio
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>