fix back-button on description diff page in case of no browser history (#13697)

Contributed by Filou Centrinov.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11893 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/views/journals/diff.html.erb

@@ -5,6 +5,9 @@
 <%= simple_format_without_paragraph @diff.to_html %>
 </div>
 
-<p><%= link_to l(:button_back), issue_path(@issue), :onclick => 'history.back(); return false;' %></p>
+<p>
+  <%= link_to(l(:button_back), issue_path(@issue),
+              :onclick => 'if (history.length > 1) {history.back(); return false;}') %>
+</p>
 
 <% html_title "#{@issue.tracker.name} ##{@issue.id}: #{@issue.subject}" %>