rtp: basedepay: Reset last used ext seqnum on discontinuities

The ext seqnum counting is reset too so keeping the old one around will
cause problems with timestamping of the next outgoing buffer.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1584>

net/rtp/src/basedepay/imp.rs

@@ -1320,7 +1320,8 @@ impl RtpBaseDepay2 {
                 pt,
                 ext_seqnum,
                 ext_rtptime,
-            })
+            });
+            state.last_used_ext_seqnum = None;
         }
 
         // Remember the initial PTS/rtp_time mapping if old-style RTSP caps are used.