modules/admin-full: only display back button in changes page if a redir url is given

modules/admin-full/luasrc/view/admin_uci/changes.htm

@@ -25,11 +25,13 @@ $Id$
 <% end %>
 
 <div class="cbi-page-actions">
+	<% local r = luci.http.formvalue("redir"); if r and #r > 0 then %>
 	<div style="float:left">
-		<form class="inline" method="get" action="<%=luci.util.pcdata(luci.http.formvalue("redir"))%>">
+		<form class="inline" method="get" action="<%=luci.util.pcdata(r)%>">
 			<input class="cbi-button cbi-button-link" style="float:left; margin:0" type="submit" value="<%:Back%>" />
 		</form>
 	</div>
+	<% end %>
 
 	<div style="text-align:right">
 		<form class="inline" method="get" action="<%=controller%>/admin/uci/apply">