dvdreadsrc: minor safety tweak to sector seeking
dvdreadsrc: minor safety tweak to sector seeking

fluendo/gst-plugins-ugly,matsu/gst-plugins-ugly,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-ugly,matsu/gst-plugins-ugly,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-ugly,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-ugly,matsu/gst-plugins-ugly,fluendo/gst-plugins-ugly,fluendo/gst-plugins-ugly,matsu/gst-plugins-ugly