Set the task arguments under the lock

I've been torturing varnish with this change for some time and was
not able to reproduce the problem.

Should fix #2719.

bin/varnishd/cache/cache_wrk.c

@@ -214,13 +214,12 @@ Pool_Task_Arg(struct worker *wrk, enum task_prio prio, task_func_t *func,
 		wrk2 = wrk;
 		retval = 0;
 	}
-	Lck_Unlock(&pp->mtx);
 	AZ(wrk2->task.func);
-
 	assert(arg_len <= WS_Reserve(wrk2->aws, arg_len));
 	memcpy(wrk2->aws->f, arg, arg_len);
 	wrk2->task.func = func;
 	wrk2->task.priv = wrk2->aws->f;
+	Lck_Unlock(&pp->mtx);
 	if (retval)
 		AZ(pthread_cond_signal(&wrk2->cond));
 	return (retval);