Fixes <https://bugs.gnu.org/62691>.
Reported by Михаил Бахтерев <mike.bakhterev@gmail.com>.
* module/ice-9/threads.scm (call-with-new-thread): Do not use 'set!'
to set object properties while the calling thread is waiting on the
new thread to initialize.
Signed-off-by: Ludovic Courtès <ludo@gnu.org>