Fix SF bug #1402308, segfault when using mmap(-1, ...)

This didn't crash on Linux, but valgrind complained.
I'm not sure if this test is valid on Windows.

Will backport.
diff --git a/Misc/NEWS b/Misc/NEWS
index d5a2361..6729ea8 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -216,6 +216,8 @@
 Extension Modules
 -----------------
 
+- Bug #1402308, (possible) segfault when using mmap.mmap(-1, ...)
+
 - Bug #1400822, _curses over{lay,write} doesn't work when passing 6 ints.
   Also fix ungetmouse() which did not accept arguments properly.
   The code now conforms to the documented signature.