Throttle: Only clear timeouts if they exist

Not sure if this is a real performance issue, but every call to clearTimeout is shown in Chrome Timeline.

Author: tkristiansen

underscore.js

@@ -744,8 +744,10 @@
       context = this;
       args = arguments;
       if (remaining <= 0 || remaining > wait) {
-        clearTimeout(timeout);
-        timeout = null;
+        if(timeout) {
+          clearTimeout(timeout);
+          timeout = null;
+        }
         previous = now;
         result = func.apply(context, args);
         if (!timeout) context = args = null;