avdevice/lavfi: Properly free an AVDictionary

It is not documented that freeing the last (and only) entry of
an AVDictionary frees the dictionary.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavdevice/lavfi.c

@@ -150,7 +150,7 @@ av_cold static int lavfi_read_header(AVFormatContext *avctx)
         if (avctx->protocol_whitelist && (ret = av_dict_set(&options, "protocol_whitelist", avctx->protocol_whitelist, 0)) < 0)
             goto end;
         ret = avio_open2(&avio, lavfi->graph_filename, AVIO_FLAG_READ, &avctx->interrupt_callback, &options);
-        av_dict_set(&options, "protocol_whitelist", NULL, 0);
+        av_dict_free(&options);
         if (ret < 0)
             goto end;
         av_bprint_init(&graph_file_pb, 0, AV_BPRINT_SIZE_UNLIMITED);