in scan_once, prevent the reading of arbitrary memory when passed a negative index

Bug reported by Guido Vranken.
diff --git a/Misc/ACKS b/Misc/ACKS
index 2fd7e33..a618002 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -1369,6 +1369,7 @@
 Frank Visser
 Johannes Vogel
 Alex Volkov
+Guido Vranken
 Martijn Vries
 Sjoerd de Vries
 Niki W. Waibel