Fix crash when passing null to clearImmediate

Summary:Passing `undefined` or `null` to `clearImmediate` caused apps to crash. It it caused because we try to find the index of the null/undefined timer when we should just do nothing when passed these values.

It is already handled properly in the other Timer functions.

**Test plan**
Calling `clearImmediate` with `undefined` or `null` should do nothing.
Closes react#6192

Differential Revision: D2987778

Pulled By: vjeux

fb-gh-sync-id: 6fd38cfa3c10012caa2afb27cbdab95df696a769
shipit-source-id: 6fd38cfa3c10012caa2afb27cbdab95df696a769

Author: janicduplessis

Libraries/JavaScriptAppEngine/System/JSTimers/JSTimers.js

@@ -106,10 +106,10 @@ var JSTimers = {
 
   clearImmediate: function(timerID) {
     JSTimers._clearTimerID(timerID);
-    JSTimersExecution.immediates.splice(
-      JSTimersExecution.immediates.indexOf(timerID),
-      1
-    );
+    var index = JSTimersExecution.immediates.indexOf(timerID);
+    if (index !== -1) {
+      JSTimersExecution.immediates.splice(index, 1);
+    }
   },
 
   cancelAnimationFrame: function(timerID) {