Use correct return value on error

src/threadqueue.c

@@ -270,7 +270,7 @@ unsigned int uc_thread_queue_length(struct uc_threadqueue *queue)
     unsigned int length;
 
     if (queue == NULL ) {
-        return -EINVAL;
+        return EINVAL;
     }
     // get the length properly
     pthread_mutex_lock(&queue->mutex);
@@ -285,7 +285,7 @@ int uc_thread_queue_clear(struct uc_threadqueue *queue,
                             void *cookie)
 {
     if (queue == NULL) {
-        return -EINVAL;
+        return EINVAL;
     }
 
     pthread_mutex_lock(&queue->mutex);