dispatch: make the test suite pass on Windows

Certain queues (e.g. the maintenance qos queue) will not have the
context setup during initialization.  This will result in a failure when
we try to dereference the context.  This allows the test suite to pass
on Windows again.

Author: compnerd

src/queue.c

@@ -6943,6 +6943,8 @@ _dispatch_root_queue_init_pthread_pool(dispatch_queue_global_t dq,
 		int pool_size, dispatch_priority_t pri)
 {
 	dispatch_pthread_root_queue_context_t pqc = dq->do_ctxt;
+	if (pqc == NULL)
+		return;
 	int thread_pool_size = DISPATCH_WORKQ_MAX_PTHREAD_COUNT;
 	if (!(pri & DISPATCH_PRIORITY_FLAG_OVERCOMMIT)) {
 		thread_pool_size = (int32_t)dispatch_hw_config(active_cpus);