avcodec/agm: Check frame dimensions

Fixes: out of array access
Fixes: 14110/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AGM_fuzzer-5649184864075776

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/agm.c

@@ -1178,6 +1178,9 @@ static int decode_frame(AVCodecContext *avctx, void *data,
             av_log(avctx, AV_LOG_ERROR, "Missing reference frame.\n");
             return AVERROR_INVALIDDATA;
         }
+        if (s->prev_frame-> width != frame->width ||
+            s->prev_frame->height != frame->height)
+            return AVERROR_INVALIDDATA;
 
         if (!(s->flags & 2)) {
             ret = av_frame_copy(frame, s->prev_frame);