timers: fix refresh inside callback

When `timers.refresh()` is called inside a callback, the timer would
incorrectly end up unrefed and thus not keep the event loop alive.

lib/internal/timers.js

@@ -469,7 +469,7 @@ function getTimerCallbacks(runNextTicks) {
           if (start === undefined)
             start = getLibuvNow();
           insert(timer, timer[kRefed], start);
-        } else {
+        } else if (!timer._idleNext && !timer._idlePrev) {
           if (timer[kRefed])
             refCount--;
           timer[kRefed] = null;

test/parallel/test-timers-refresh-in-callback.js

@@ -0,0 +1,16 @@
+// Flags: --expose-internals
+
+'use strict';
+
+const common = require('../common');
+
+// This test checks whether a refresh called inside the callback will keep
+// the event loop alive to run the timer again.
+
+let didCall = false;
+const timer = setTimeout(common.mustCall(() => {
+  if (!didCall) {
+    didCall = true;
+    timer.refresh();
+  }
+}, 2), 1);