decoder/ffmpeg: fix double free bug

From the avformat_open_input() API documentation:

 "Note that a user-supplied AVFormatContext will be freed on failure."

https://bugs.musicpd.org/view.php?id=4607

src/decoder/plugins/FfmpegDecoderPlugin.cxx

@@ -71,7 +71,6 @@ FfmpegOpenInput(AVIOContext *pb,
 
 	int err = avformat_open_input(&context, filename, fmt, nullptr);
 	if (err < 0) {
-		avformat_free_context(context);
 		SetFfmpegError(error, err, "avformat_open_input() failed");
 		return nullptr;
 	}