mpegtslivesrc: Fix mismatch between internal / external time usage

Previously the internal time was stored as base offset for calculating
the external time from the PCR, which resulted in disconts being
detected wrongly.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1933>

net/mpegtslive/src/mpegtslive/imp.rs

@@ -460,7 +460,7 @@ impl State {
                 );
                 new_pcr = MpegTsPcr::new(pcr);
                 self.base_pcr = Some(new_pcr);
-                self.base_external = Some(observation_internal);
+                self.base_external = Some(base_external);
                 self.discont_pending = true;
             }
         } else {