avcodec/svq3: Dont memcpy AVFrame

This avoids out of array accesses

Fixes: asan_heap-uaf_21f42e4_9_asan_heap-uaf_21f42e4_278_gl2.mov

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
