stylecontext: Make sure style is valid when looking up style properties

Otherwise, the validation may happen vey automatically some time during
the style property and that will most likely cause a crash.

https://bugzilla.gnome.org/show_bug.cgi?id=756338

gtk/gtkstylecontext.c

@@ -1557,6 +1557,9 @@ _gtk_style_context_peek_style_property (GtkStyleContext *context,
 
   priv = context->priv;
 
+  /* ensure the style cache is valid by forcing a validation */
+  gtk_style_context_lookup_style (context);
+
   key.widget_type = widget_type;
   key.pspec = pspec;