BUILD: task: silence a build warning with threads disabled

The compiler doesn't know that a random value based on global.nbthread
is necessarily smaller than MAX_THREADS, and when picking a random
thread number while single-threaded it complains that new_tid 1 is
out of bounds for the array. In fact all this is dead code in this
case.

Let's tell it about it to silence the warning.

src/task.c

@@ -395,6 +395,8 @@ void wake_expired_tasks()
 
 					if (new_tid == tid)
 						continue;
+
+					ASSUME(new_tid < MAX_THREADS);
 					if (ha_thread_ctx[new_tid].rq_total * 2 < th_ctx->rq_total) {
 						int cur_state;
 						do {