lavf,lavc: free avoptions in a generic way.

It's simpler and less error-prone.

Fixes some memleaks along the way.

libavcodec/utils.c

@@ -783,6 +783,9 @@ av_cold int avcodec_close(AVCodecContext *avctx)
         avctx->codec->close(avctx);
     avcodec_default_free_buffers(avctx);
     avctx->coded_frame = NULL;
+    if (avctx->codec->priv_class)
+        av_opt_free(avctx->priv_data);
+    av_opt_free(avctx);
     av_freep(&avctx->priv_data);
     if(avctx->codec && avctx->codec->encode)
         av_freep(&avctx->extradata);