avoid accessing nonexistant array key. fixes #1165

inc/html.php

@@ -1409,7 +1409,13 @@ function html_diff_navigation($pagelog, $type, $l_rev, $r_rev) {
 
     // last timestamp is not in changelog, retrieve timestamp from metadata
     // note: when page is removed, the metadata timestamp is zero
-    $r_rev = $r_rev ? $r_rev : $INFO['meta']['last_change']['date'];
+    if(!$r_rev) {
+        if(isset($INFO['meta']['last_change']['date'])) {
+            $r_rev = $INFO['meta']['last_change']['date'];
+        } else {
+            $r_rev = 0;
+        }
+    }
 
     //retrieve revisions with additional info
     list($l_revs, $r_revs) = $pagelog->getRevisionsAround($l_rev, $r_rev);