lavc: Prevent opening of experimental codecs if strict_std_compliance > FF_COMPLIANCE_EXPERIMENTAL.

Some of these encoders may produce invalid bitstreams, which should not
be done without the user knowing.
Some of these decoders may be unfinished and may contain security issues.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/utils.c

@@ -703,6 +703,13 @@ int attribute_align_arg avcodec_open2(AVCodecContext *avctx, AVCodec *codec, AVD
     if ((ret = av_opt_set_dict(avctx, &tmp)) < 0)
         goto free_and_end;
 
+    if (codec->capabilities & CODEC_CAP_EXPERIMENTAL)
+        if (avctx->strict_std_compliance > FF_COMPLIANCE_EXPERIMENTAL) {
+            av_log(avctx, AV_LOG_ERROR, "Codec is experimental but experimental codecs are not enabled, see -strict -2\n");
+            ret = -1;
+            goto free_and_end;
+        }
+
     //We only call avcodec_set_dimensions() for non h264 codecs so as not to overwrite previously setup dimensions
     if(!( avctx->coded_width && avctx->coded_height && avctx->width && avctx->height && avctx->codec_id == CODEC_ID_H264)){
     if(avctx->coded_width && avctx->coded_height)