BUG/MEDIUM: thread: check stopping thread against local bit and not global one

Commit ef422ced9 ("MEDIUM: thread: make stopping_threads per-group and add
stopping_tgroups") moved the stopping_threads mask to per-group, but one
test in the loop preserved its global value instead, resulting in stopping
threads never sleeping on stop and eating 100% CPU until all were stopped.

No backport is needed.
diff --git a/src/haproxy.c b/src/haproxy.c
index 145c19a..e8c209e 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -2818,7 +2818,7 @@
 
 			if (stopping) {
 				/* stop muxes before acknowledging stopping */
-				if (!(tg_ctx->stopping_threads & tid_bit)) {
+				if (!(tg_ctx->stopping_threads & ti->ltid_bit)) {
 					task_wakeup(mux_stopping_data[tid].task, TASK_WOKEN_OTHER);
 					wake = 1;
 				}