timers: fix a bug in error handling

When a timeout within a list of timeouts (that consists of only
that specific timeout) throws during its execution, it's possible
for the TimerWrap handle to become shared between both that list
and an unref'd timeout created in the future. This fixes the bug
by extending error handling in timeout execution to check for
whether the current list is empty and if so do cleanup on it.

PR-URL: https://github.com/nodejs/node/pull/20497
Fixes: https://github.com/nodejs/node/issues/19970


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Shingo Inoue <leko.noor@gmail.com>