unicodeobject.c: fix a compiler warning on Windows 64 bits

Objects/unicodeobject.c

@@ -7015,7 +7015,8 @@ decode_code_page_errors(UINT code_page,
     assert(outsize <= PyUnicode_WSTR_LENGTH(*v));
     if (unicode_resize(v, outsize) < 0)
         goto error;
-    ret = in - startin;
+    /* (in - startin) <= size and size is an int */
+    ret = Py_SAFE_DOWNCAST(in - startin, Py_ssize_t, int);
 
 error:
     Py_XDECREF(encoding_obj);