dvdread: Also reset the first_seek var in start()
dvdread: Also reset the first_seek var in start()

Make the initial chapter seek work across reuse.

https://bugzilla.gnome.org/show_bug.cgi?id=453322

GStreamer/gst-plugins-ugly,GStreamer/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,Kurento/gst-plugins-ugly,Kurento/gst-plugins-ugly,shelsonjava/gst-plugins-ugly,shelsonjava/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,shelsonjava/gst-plugins-ugly,GStreamer/gst-plugins-ugly,Kurento/gst-plugins-ugly