h264: check that an IDR NAL only contains I slices

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC:libav-stable@libav.org

libavcodec/h264.c

@@ -3383,6 +3383,12 @@ static int decode_slice_header(H264Context *h, H264Context *h0)
     h->slice_type     = slice_type;
     h->slice_type_nos = slice_type & 3;
 
+    if (h->nal_unit_type  == NAL_IDR_SLICE &&
+        h->slice_type_nos != AV_PICTURE_TYPE_I) {
+        av_log(h->avctx, AV_LOG_ERROR, "A non-intra slice in an IDR NAL unit.\n");
+        return AVERROR_INVALIDDATA;
+    }
+
     // to make a few old functions happy, it's wrong though
     h->pict_type = h->slice_type;