avcodec/rasc: unref both frames on reinit

Fixes: integer overflow
Fixes: inconsistent frame dimensions
Fixes: 10454/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_RASC_fuzzer-5656301162463232

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/rasc.c

@@ -95,10 +95,10 @@ static int init_frames(AVCodecContext *avctx)
     int ret;
 
     av_frame_unref(s->frame1);
+    av_frame_unref(s->frame2);
     if ((ret = ff_get_buffer(avctx, s->frame1, 0)) < 0)
         return ret;
 
-    av_frame_unref(s->frame2);
     if ((ret = ff_get_buffer(avctx, s->frame2, 0)) < 0)
         return ret;