Added max heartbeat interval to prevent integer overflow

When using big session timeout values, the interval value might
overflow and cause the setInterval() call to ping the server in a loop
without any delay.

This fix adds a maximum ping interval of 24 hours.

Forward port of 00ec5fc193 from stable6.

core/js/js.js

@@ -968,6 +968,8 @@ function initCore() {
 	 * time out
 	 */
 	function initSessionHeartBeat(){
+		// max interval in seconds set to 24 hours
+		var maxInterval = 24 * 3600;
 		// interval in seconds
 		var interval = 900;
 		if (oc_config.session_lifetime) {
@@ -977,6 +979,9 @@ function initCore() {
 		if (interval < 60) {
 			interval = 60;
 		}
+		if (interval > maxInterval) {
+			interval = maxInterval;
+		}
 		var url = OC.generateUrl('/heartbeat');
 		setInterval(function(){
 			$.post(url);