Don't break kolab_storage_cache::select() return data if reading one single cache entry fails

plugins/libkolab/lib/kolab_storage_cache.php

@@ -491,7 +491,7 @@ class kolab_storage_cache
                 else if ($fetchall && ($object = $this->_unserialize($sql_arr))) {
                     $result[] = $object;
                 }
-                else {
+                else if (!$fetchall) {
                     // only add msguid to dataset index
                     $result[] = $sql_arr;
                 }