[pitivi] elements: Fix backtrace when showing tooltip

[Thibault Saunier <tsaunier src gnome org>]

commit 673910407a20fb4be925095c06033efd091c34e5
Author: Alexandru Băluț <alexandru balut gmail com>
Date:   Sun Mar 13 09:17:52 2016 +0100

    elements: Fix backtrace when showing tooltip
    
    When hovering the line at the very left, the assert res fails in edge
    cases. You could also blame double numbers.
    
    This can be easily avoided by making sure the x is in range.
    
    Reviewed-by: Mathieu Duponchelle <mathieu duponchelle epitech eu>
    Differential Revision: https://phabricator.freedesktop.org/D827

 pitivi/timeline/elements.py |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/pitivi/timeline/elements.py b/pitivi/timeline/elements.py
index 85e8c94..079dbe4 100644
--- a/pitivi/timeline/elements.py
+++ b/pitivi/timeline/elements.py
@@ -329,14 +329,15 @@ class KeyframeCurve(FigureCanvas, Loggable):
 
     def __setTooltip(self, event):
         if event.xdata:
-            res, value = self.__source.control_source_get_value(event.xdata)
+            xdata = max(self.__line_xs[0], min(event.xdata, self.__line_xs[-1]))
+            res, value = self.__source.control_source_get_value(xdata)
             assert res
             pmin = self.__paramspec.minimum
             pmax = self.__paramspec.maximum
             value = value * (pmax - pmin) + pmin
             self.set_tooltip_markup(_("Property: %s\nTimestamp: %s\nValue: %s")
                                     % (self.__propertyName,
-                                       Gst.TIME_ARGS(event.xdata),
+                                       Gst.TIME_ARGS(xdata),
                                        "{:.3f}".format(value)))
 
     def __resetTooltip(self):