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.

(merge with 3.2)

Misc/NEWS

@@ -10,6 +10,8 @@ What's New in Python 3.3.0 Release Candidate 1?
 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.
 

Modules/main.c

@@ -604,7 +604,7 @@ Py_Main(int argc, wchar_t **argv)
        script. */
     if ((p = Py_GETENV("PYTHONEXECUTABLE")) && *p != '\0') {
         wchar_t* buffer;
-        size_t len = strlen(p);
+        size_t len = strlen(p) + 1;
 
         buffer = malloc(len * sizeof(wchar_t));
         if (buffer == NULL) {