GstRTSPMountPoints: Remove any existing factory before adding a new one
The documentation of gst_rtsp_mount_points_add_factory() says "Any previous mount point will be freed" which was true when it was implemented using a GHashTable. But in 2012 it got rewrote using a GSequence and since then it could have 2 factories for the same path. Which one gets used is random, depending on the sorting order of 2 identical items.