Qt: avoid a crash on displaying the chapters for some mp4

(cherry picked from commit 491b918f)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/qt4/util/input_slider.cpp

@@ -260,7 +260,7 @@ void SeekSlider::mouseMoveEvent( QMouseEvent *event )
                     if ( event->x() >= x )
                         i_selected = i + ( ( b_startsnonzero )? 1 : 0 );
                 }
-                if ( i_selected >= 0 )
+                if ( i_selected >= 0 && i_selected < points.size() )
                     chapterLabel = points.at( i_selected ).name;
         }