css: Avoid a possible memory leak

Currently, style->variables is always NULL when we get here, but
better to be safe than sorry, and clear the fields before we
overwrite it.

gtk/gtkcssstaticstyle.c

@@ -865,6 +865,7 @@ gtk_css_lookup_resolve (GtkCssLookup      *lookup,
     }
   else if (parent_style && parent_style->variables)
     {
+      g_clear_pointer (&style->variables, gtk_css_variable_set_unref);
       style->variables = gtk_css_variable_set_ref (parent_style->variables);
     }