Super Cache: check that plugin links is an array before adding Settings link

[donnchawp]

This is a very odd one, as it was only ever reported once here. The user reported an error in the code that adds the Settings link to the Plugins page entry for this plugin.
As this is a non critical part of the plugin, I'm tempted to remove the code entirely because when the plugin is installed first, an admin notice is displayed prompting the user to go to the settings page.
Another fix will be a small check on the $links array to make sure it is an array in the wp_cache_plugin_actions function in wp-cache.php