Qt: check colorDialog return function

Close #5679
(cherry picked from commit bb12c0023452e86a26e407d6e728d961e4544f5e)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/qt4/components/preferences_widgets.cpp

@@ -1175,10 +1175,13 @@ int ColorConfigControl::getValue() const
 void ColorConfigControl::selectColor()
 {
     QColor color = QColorDialog::getColor( QColor( i_color ) );
+    if( color.isValid() )
+    {
         i_color = (color.red() << 16) + (color.green() << 8) + color.blue();
 
         color_px->fill( QColor( i_color ) );
         color_but->setIcon( QIcon( *color_px ) );
+    }
 }