pthreadpool_get_threads_count: support NULL threadpool
diff --git a/src/threadpool-pthreads.c b/src/threadpool-pthreads.c
index f7d4dfb..ff0817e 100644
--- a/src/threadpool-pthreads.c
+++ b/src/threadpool-pthreads.c
@@ -368,7 +368,11 @@
 }
 
 size_t pthreadpool_get_threads_count(struct pthreadpool* threadpool) {
-	return threadpool->threads_count;
+	if (threadpool == NULL) {
+		return 1;
+	} else {
+		return threadpool->threads_count;
+	}
 }
 
 void pthreadpool_compute_1d(