CLEANUP: thread: also remove a thread's bit from stopping_threads on stop

As much as possible we should take care of not leaving bits from stopped
threads in shared thread masks. It can avoid issues like the previous
fix and will also make debugging less confusing.
diff --git a/src/haproxy.c b/src/haproxy.c
index f86e3b5..85467a5 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -2988,6 +2988,8 @@
 #ifdef USE_THREAD
 	if (!_HA_ATOMIC_AND_FETCH(&ha_tgroup_info[ti->tgid-1].threads_enabled, ~ti->ltid_bit))
 		_HA_ATOMIC_AND(&all_tgroups_mask, ~tg->tgid_bit);
+	if (!_HA_ATOMIC_AND_FETCH(&tg_ctx->stopping_threads, ~ti->ltid_bit))
+		_HA_ATOMIC_AND(&stopping_tgroup_mask, ~tg->tgid_bit);
 	_HA_ATOMIC_AND(&all_threads_mask, ~tid_bit);
 	if (tid > 0)
 		pthread_exit(NULL);