avformat/fifo: Free dict on av_dict_copy() failure

av_dict_copy() puts the onus on the caller to clean up dst on failure;
it can be nonempty if copying a later entry of src fails after having
successfully copied an earlier entry.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/fifo.c

@@ -129,7 +129,7 @@ static int fifo_thread_write_header(FifoThreadContext *ctx)
 
     ret = av_dict_copy(&format_options, fifo->format_options, 0);
     if (ret < 0)
-        return ret;
+        goto end;
 
     ret = ff_format_output_open(avf2, avf->url, &format_options);
     if (ret < 0) {