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: https://github.com/nodejs/node/pull/18579


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