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>
(cherry picked from commit 075a165d27)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/svq3.c

@@ -1176,7 +1176,7 @@ static int svq3_decode_frame(AVCodecContext *avctx, void *data,
 
     h->cur_pic_ptr = s->cur_pic;
     av_frame_unref(&h->cur_pic.f);
-    h->cur_pic     = *s->cur_pic;
+    memcpy(&h->cur_pic.tf, &s->cur_pic->tf, sizeof(h->cur_pic) - offsetof(H264Picture, tf));
     ret = av_frame_ref(&h->cur_pic.f, &s->cur_pic->f);
     if (ret < 0)
         return ret;