avcodec/flicvideo: Optimize and Simplify FLI_COPY in flic_decode_frame_24BPP() by using bytestream2_get_buffer()

Fixes: Timeout (31sec  -> 22sec)
Fixes: 16217/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FLIC_fuzzer-5658084189405184

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Tomas Härdin <tjoppen@acc.umu.se>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit e301736862)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/flicvideo.c

@@ -1024,14 +1024,7 @@ static int flic_decode_frame_24BPP(AVCodecContext *avctx,
                 for (y_ptr = 0; y_ptr < s->frame->linesize[0] * s->avctx->height;
                      y_ptr += s->frame->linesize[0]) {
 
-                    pixel_countdown = s->avctx->width;
-                    pixel_ptr = 0;
-                    while (pixel_countdown > 0) {
-                        pixel = bytestream2_get_le24(&g2);
-                        AV_WL24(&pixels[y_ptr + pixel_ptr], pixel);
-                        pixel_ptr += 3;
-                        pixel_countdown--;
-                    }
+                    bytestream2_get_buffer(&g2, pixels + y_ptr, 3*s->avctx->width);
                     if (s->avctx->width & 1)
                         bytestream2_skip(&g2, 3);
                 }