->getevents() should take unsigned args

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/engines/null.c b/engines/null.c
index bbb4e8a..85d6847 100644
--- a/engines/null.c
+++ b/engines/null.c
@@ -26,8 +26,9 @@
 	return nd->io_us[event];
 }
 
-static int fio_null_getevents(struct thread_data *td, int min_events,
-			      int fio_unused max, struct timespec fio_unused *t)
+static int fio_null_getevents(struct thread_data *td, unsigned int min_events,
+			      unsigned int fio_unused max,
+			      struct timespec fio_unused *t)
 {
 	struct null_data *nd = td->io_ops->data;
 	int ret = 0;