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
diff --git a/Misc/NEWS b/Misc/NEWS
index 3913f94..e44219a 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -10,6 +10,9 @@
 Library
 -------
 
+- Fix arbitrary memory access in JSONDecoder.raw_decode with a negative second
+  parameter. Bug reported by Guido Vranken.
+
 - Issue #21082: In os.makedirs, do not set the process-wide umask. Note this
   changes behavior of makedirs when exist_ok=True.