make sure we don't overflow the workers array

src/core/workers.c

@@ -2213,6 +2213,7 @@ int starpu_worker_get_stream_workerids(unsigned devid, int *workerids, enum star
 
 void starpu_worker_get_sched_condition(int workerid, starpu_pthread_mutex_t **sched_mutex, starpu_pthread_cond_t **sched_cond)
 {
+	STARPU_ASSERT(workerid >= 0 && workerid < STARPU_NMAXWORKERS);
 	*sched_cond = &_starpu_config.workers[workerid].sched_cond;
 	*sched_mutex = &_starpu_config.workers[workerid].sched_mutex;
 }