timers: be more defensive with intervals

It's possible for user-code to flip an existing timeout to
be an interval during its execution, in which case the
current code would crash due to start being undefined. Fix
this by providing a default start value within rearm.

PR-URL: #18579
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

lib/timers.js

@@ -442,8 +442,7 @@ function ontimeout(timer, start) {
     rearm(timer, start);
 }
 
-
-function rearm(timer, start) {
+function rearm(timer, start = TimerWrap.now()) {
   // // Do not re-arm unenroll'd or closed timers.
   if (timer._idleTimeout === -1) return;
 

test/parallel/test-timers-timeout-to-interval.js

@@ -0,0 +1,12 @@
+'use strict';
+const common = require('../common');
+
+// This isn't officially supported but nonetheless is something that is
+// currently possible and as such it shouldn't cause the process to crash
+
+const t = setTimeout(common.mustCall(() => {
+  if (t._repeat) {
+    clearInterval(t);
+  }
+  t._repeat = true;
+}, 2), 1);