avcodec/vc1dec: Check field_mode for sprites

Fixes: Out of array read
Fixes: 19263/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_VC1IMAGE_fuzzer-5389219325542400

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/vc1dec.c

@@ -854,7 +854,12 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
         ret = AVERROR_INVALIDDATA;
         goto err;
     }
-
+    if ((avctx->codec_id == AV_CODEC_ID_WMV3IMAGE || avctx->codec_id == AV_CODEC_ID_VC1IMAGE)
+        && v->field_mode) {
+        av_log(v->s.avctx, AV_LOG_ERROR, "Sprite decoder: expected Frames not Fields\n");
+        ret = AVERROR_INVALIDDATA;
+        goto err;
+    }
     if ((s->mb_height >> v->field_mode) == 0) {
         av_log(v->s.avctx, AV_LOG_ERROR, "image too short\n");
         ret = AVERROR_INVALIDDATA;