Fix cleaning up external feeds that we don't need anymore, if someone has...

Fix cleaning up external feeds that we don't need anymore, if someone has added an RSS blocktype and hasn't actually put a feed in it.

htdocs/blocktype/externalfeed/lib.php

@@ -186,7 +186,9 @@ class PluginBlocktypeExternalfeed extends SystemBlocktype {
         if ($instances = get_records_array('block_instance', 'blocktype', 'externalfeed')) {
             foreach ($instances as $block) {
                 $data = unserialize($block->configdata);
-                $ids[$data['feedid']] = true;
+                if ($data['feedid']) {
+                    $ids[$data['feedid']] = true;
+                }
             }
         }
         if (count($ids) == 0) {