[pitivi] timeline: Fix assertion in the wavefor generation code

[Thibault Saunier <tsaunier src gnome org>]

commit b9675bd1147ac7be7cd7f96c38171352f824f5bc
Author: Thibault Saunier <tsaunier gnome org>
Date:   Thu Jul 21 10:04:41 2016 -0400

    timeline: Fix assertion in the wavefor generation code
    
    We should not query position on a pipeline that is in READY state
    
    Fixes T7510
    
    Reviewed-by: Alex Băluț <alexandru balut gmail com>
    Differential Revision: https://phabricator.freedesktop.org/D1193

 pitivi/timeline/previewers.py |    9 +++++----
 1 files changed, 5 insertions(+), 4 deletions(-)
---
diff --git a/pitivi/timeline/previewers.py b/pitivi/timeline/previewers.py
index 881cab2..1fe8ea9 100644
--- a/pitivi/timeline/previewers.py
+++ b/pitivi/timeline/previewers.py
@@ -893,11 +893,12 @@ class PipelineCpuAdapter(Loggable):
         if usage_percent >= WAVEFORMS_CPU_USAGE:
             if self.rate < 0.1:
                 if not self.ready:
-                    self.ready = True
-                    self.pipeline.set_state(Gst.State.READY)
-                    res, self.lastPos = self.pipeline.query_position(
+                    res, position = self.pipeline.query_position(
                         Gst.Format.TIME)
-                    assert res
+                    if res:
+                        self.lastPos = position
+                    self.pipeline.set_state(Gst.State.READY)
+                    self.ready = True
                 return True
 
             if self.rate > 0.0: