rmdemux: Mark demuxer as "running" in push-mode
rmdemux: Mark demuxer as "running" in push-mode

Allows queries (and other code) to be answered when working in push-mode.

Running would only be set to TRUE accordingly when working in pull-mode
once it had read header/index.

Fixes #680185

GrokImageCompression/gst-plugins-ugly,krieger-od/gst-plugins-ugly,jpakkane/gstreamer-plugins-ugly,shelsonjava/gst-plugins-ugly,collects/gst-plugins-ugly,krieger-od/gst-plugins-ugly,reynaldo-samsung/gst-plugins-ugly,Lachann/gst-plugins-ugly,shelsonjava/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,surround-io/gst-plugins-ugly,GStreamer/gst-plugins-ugly,surround-io/gst-plugins-ugly,collects/gst-plugins-ugly,jar1karp/gst-plugins-ugly,freedesktop-unofficial-mirror/gstreamer__gst-plugins-ugly,freedesktop-unofficial-mirror/gstreamer__gst-plugins-ugly,GStreamer/gst-plugins-ugly,Kurento/gst-plugins-ugly,cablelabs/gst-plugins-ugly,GStreamer/gst-plugins-ugly,reynaldo-samsung/gst-plugins-ugly,krieger-od/gst-plugins-ugly,collects/gst-plugins-ugly,sebras/gst-plugins-ugly,sebras/gst-plugins-ugly,jpakkane/gstreamer-plugins-ugly,sebras/gst-plugins-ugly,jar1karp/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,reynaldo-samsung/gst-plugins-ugly,freedesktop-unofficial-mirror/gstreamer__gst-plugins-ugly,cablelabs/gst-plugins-ugly,Kurento/gst-plugins-ugly,shelsonjava/gst-plugins-ugly,jpakkane/gstreamer-plugins-ugly,GrokImageCompression/gst-plugins-ugly,Kurento/gst-plugins-ugly,cablelabs/gst-plugins-ugly,Lachann/gst-plugins-ugly,krieger-od/gst-plugins-ugly,collects/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,Lachann/gst-plugins-ugly,surround-io/gst-plugins-ugly,jar1karp/gst-plugins-ugly,StreamUtils/gst-plugins-ugly