timers: fix handling of cleared immediates

If current immediate has no callback, move on to the next one in
the queue.

Fixes: https://github.com/nodejs/node/issues/9756
PR-URL: https://github.com/nodejs/node/pull/9759
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

lib/timers.js

@@ -580,8 +580,10 @@ function processImmediate() {
   while (immediate) {
     domain = immediate.domain;
 
-    if (!immediate._onImmediate)
+    if (!immediate._onImmediate) {
+      immediate = immediate._idleNext;
       continue;
+    }
 
     if (domain)
       domain.enter();

test/parallel/test-timers-regress-GH-9765.js

@@ -0,0 +1,23 @@
+'use strict';
+const common = require('../common');
+
+// This test ensures that if an Immediate callback clears subsequent
+// immediates we don't get stuck in an infinite loop.
+//
+// If the process does get stuck, it will be timed out by the test
+// runner.
+//
+// Ref: https://github.com/nodejs/node/issues/9756
+
+setImmediate(common.mustCall(function() {
+  clearImmediate(i2);
+  clearImmediate(i3);
+}));
+
+const i2 = setImmediate(function() {
+  common.fail('immediate callback should not have fired');
+});
+
+const i3 = setImmediate(function() {
+  common.fail('immediate callback should not have fired');
+});