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As I can see, the proper fix is to replace

static void
w_long(long x, WFILE *p)
{
    w_byte((int)( x      & 0xff), p);
    w_byte((int)((x>> 8) & 0xff), p);
    w_byte((int)((x>>16) & 0xff), p);
   w_byte((int)((x>>24) & 0xff), p);
}

with

static void
w_long(long x, WFILE *p)
{
    w_byte((char)( x      & 0xff), p);
    w_byte((char)((x>> 8) & 0xff), p);
    w_byte((char)((x>>16) & 0xff), p);
    w_byte((char)((x>>24) & 0xff), p);
}

and similar for the w_short() function.
Even safer would be to use the Py_SAFE_DOWNCAST 
macro also in the w_byte macro.
Attached a patch for marshal.c.