Emit ::style-set after setting priv->style

This was inadvertedly changed with an optimization a while ago,
and can lead to application crashes.

https://bugzilla.gnome.org/show_bug.cgi?id=773029

gtk/gtkwidget.c

@@ -16621,8 +16621,8 @@ void
 _gtk_widget_set_style (GtkWidget *widget,
                        GtkStyle  *style)
 {
-  g_signal_emit (widget, widget_signals[STYLE_SET], 0, widget->priv->style);
   widget->priv->style = style;
+  g_signal_emit (widget, widget_signals[STYLE_SET], 0, widget->priv->style);
 }
 
 GtkActionMuxer *