clear gstreamer timer when removing it

fixes warnings about cancelling a non-existing source

xpra/server/window/video_compress.py

@@ -676,6 +676,7 @@ def gstreamer_nodamage(self) -> None:
     def cancel_gstreamer_timer(self) -> None:
         gt = self.gstreamer_timer
         if gt:
+            self.gstreamer_timer = 0
             GLib.source_remove(gt)
 
     def start_gstreamer_pipeline(self) -> bool: