fix security issue 2: imageop's poor validation of arguments could result in segfaults

patch by Victor Stinner
reviewed by myself and Brett
diff --git a/Misc/NEWS b/Misc/NEWS
index bbbeb91..1b08d76 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -54,6 +54,9 @@
 Extension Modules
 -----------------
 
+- Security Issue #2: imageop did not validate arguments correctly and could
+  segfault as a result.
+
 - Issue #3886: Possible integer overflows in the _hashopenssl module were
   closed.