Fix for issue 15716: interpreter could crash when PYTHONEXECUTABLE was set on Mac OS X.

This is due to an off-by-one error: the allocated buffer didn't have room for a NUL
character at the end of the mbstowcs result.
diff --git a/Misc/NEWS b/Misc/NEWS
index 87a89fb..a9c9695 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -10,6 +10,8 @@
 Core and Builtins
 -----------------
 
+- Issue #15761: Fix crash when PYTHONEXECUTABLE is set on Mac OS X.
+
 - Issue #15726: Fix incorrect bounds checking in PyState_FindModule.
   Patch by Robin Schreiber.