cssvalue: Handle named colors not resolving

If a named color references a nonexistant named color, we didn't catch
that error and ended up crashing on a NULL-dereference.
This crashed Boxes, because its CSS referenced values from the theme
that didn't exist in any theme.

gtk/gtkcsscolorvalue.c

@@ -166,6 +166,8 @@ _gtk_css_color_value_resolve (GtkCssValue             *color,
 	  return NULL;
 
         value = _gtk_css_color_value_resolve (named, provider, current, current_deps, dependencies);
+	if (value == NULL)
+	  return NULL;
       }
 
       break;