Add sanity check when auto-unsubscribing non-existing folders

program/lib/Roundcube/rcube_imap.php

@@ -2931,12 +2931,17 @@ class rcube_imap extends rcube_storage
         else {
             // unsubscribe non-existent folders, remove them from the list
             if (!empty($result) && $name == '*') {
-                $existing    = $this->list_folders($root, $name);
-                $nonexisting = array_diff($result, $existing);
-                $result      = array_diff($result, $nonexisting);
+                $existing = $this->list_folders($root, $name);
 
-                foreach ($nonexisting as $folder) {
-                    $this->conn->unsubscribe($folder);
+                // Try to make sure the list of existing folders is not malformed,
+                // we don't want to unsubscribe existing folders on error
+                if (is_array($existing) && (!empty($root) || count($existing) > 1)) {
+                    $nonexisting = array_diff($result, $existing);
+                    $result      = array_diff($result, $nonexisting);
+
+                    foreach ($nonexisting as $folder) {
+                        $this->conn->unsubscribe($folder);
+                    }
                 }
             }
         }