bug#55356: join-thread unusable if timeout was hit

[angry rectangle]

If join-thread is called after a previous call to join-thread hits the timeout, 
a "mutex already locked by thread" error is thrown.

Working example:

(use-modules (ice-9 threads))
(define t (call-with-new-thread
           (lambda _
             (let r ()
               (sleep 1)
               (r)))))
(join-thread t (current-time))
(join-thread t (current-time)) ;; exception here

my guile version: latest git. cc455976813ab94de121395982435430874cbf58
my OS: guix on amd64