splitmuxsrc: Protect total_duration state variable with the object lock.
splitmuxsrc: Protect total_duration state variable with the object lock.

Prevent deadlocks from downstream querying duration from the streaming thread.

vatavuserban/gst-plugins-good,chamois94/gst-plugins-good,ikonst/gst-plugins-good,kittee/gst-plugins-good,shelsonjava/gst-plugins-good,GrokImageCompression/gst-plugins-good,sebras/gst-plugins-good,kittee/gst-plugins-good,stfl/gst-plugins-good,GStreamer/gst-plugins-good,shelsonjava/gst-plugins-good,hizukiayaka/gst-plugins-good,rawoul/gst-plugins-good,StreamUtils/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good,froggatt/gst-plugins-good-m,pexip/gst-plugins-good,hizukiayaka/gst-plugins-good,froggatt/gst-plugins-good-m,sebras/gst-plugins-good,kittee/gst-plugins-good,chamois94/gst-plugins-good,kittee/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,vatavuserban/gst-plugins-good,GStreamer/gst-plugins-good,chamois94/gst-plugins-good,Kurento/gst-plugins-good,shelsonjava/gst-plugins-good,stfl/gst-plugins-good,vatavuserban/gst-plugins-good,shelsonjava/gst-plugins-good,pexip/gst-plugins-good,froggatt/gst-plugins-good-m,stfl/gst-plugins-good,hizukiayaka/gst-plugins-good,StreamUtils/gst-plugins-good,vatavuserban/gst-plugins-good,GStreamer/gst-plugins-good,hizukiayaka/gst-plugins-good,rawoul/gst-plugins-good,GrokImageCompression/gst-plugins-good,stfl/gst-plugins-good,ikonst/gst-plugins-good,chamois94/gst-plugins-good,sebras/gst-plugins-good,pexip/gst-plugins-good,Kurento/gst-plugins-good,GrokImageCompression/gst-plugins-good,Kurento/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,rawoul/gst-plugins-good,ikonst/gst-plugins-good,ikonst/gst-plugins-good,froggatt/gst-plugins-good-m,rawoul/gst-plugins-good,sebras/gst-plugins-good,pexip/gst-plugins-good