[pitivi] validate: Avoid failing when Wnck is missing

[Thibault Saunier <tsaunier src gnome org>]

commit 0bf76a8e5e9fa7447d30edc1084ebd3c4eee3e2e
Author: Thibault Saunier <tsaunier gnome org>
Date:   Wed Aug 10 20:59:06 2016 -0400

    validate: Avoid failing when Wnck is missing

 pitivi/utils/validate.py |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/pitivi/utils/validate.py b/pitivi/utils/validate.py
index 0ab9e87..e54be74 100644
--- a/pitivi/utils/validate.py
+++ b/pitivi/utils/validate.py
@@ -73,7 +73,7 @@ if GstValidate:
                     gi.require_version('Wnck', '3.0')
                     from gi.repository import Wnck
                     Wnck.Screen.get_default().connect("window-opened", self._windowOpenedCb)
-                except ImportError:
+                except (ImportError, ValueError):
                     print("Wnck not present on the system,"
                           " not checking the sink does not open a new window")
                     pass