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 b950474..a932074 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -1139,6 +1139,7 @@
 Johannes Vogel
 Martijn Vries
 Sjoerd de Vries
+Guido Vranken
 Niki W. Waibel
 Wojtek Walczak
 Charles Waldman