Use the right size. On 64-bit platforms, `GstMetaInfo` is larger than `GstRtmpMeta`, which masked this bug. On 32-bit platforms, it causes crashes. Thanks to @maxatka for discovering this. Fixes: https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/issues/1721 Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2553>