[Bug libstdc++/94996] jthread should stop and join the associated thread before being assigned.

[hotwatermorning at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94996

--- Comment #1 from hotwatermorning at gmail dot com ---
Note: The expected behavior is that the associated thread is stopped and joined
at `th = std::jthread();` and the following assertion passes, but the current
implementation aborts at `th = std::jthread();` because the internal _M_thread
data-member is move-assigned while joinable() == true.