liblzma: mt dec: Simplify by removing the THR_STOP state

liblzma: mt dec: Simplify by removing the THR_STOP state

The main thread can directly set THR_IDLE in threads_stop() which is
called when errors are detected. threads_stop() won't return the stopped
threads to the pool or free the memory pointed by thr->in anymore, but
it doesn't matter because the existing workers won't be reused after
an error. The resources will be cleaned up when threads_end() is
called (reinitializing the decoder always calls threads_end()).

Reviewed-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Thanks-to: Sam James <sam@gentoo.org>
(cherry picked from commit c0c835964dfaeb2513a3c0bdb642105152fe9f34)