bpo-36946: Fix possible signed integer overflow when handling slices. (GH-13375)

The final addition (cur += step) may overflow, so use size_t for "cur".
"cur" is always positive (even for negative steps), so it is safe to use
size_t here.

Co-Authored-By: Martin Panter <vadmium+py@gmail.com>

Objects/bytesobject.c

@@ -1677,7 +1677,8 @@ bytes_subscript(PyBytesObject* self, PyObject* item)
         return PyLong_FromLong((unsigned char)self->ob_sval[i]);
     }
     else if (PySlice_Check(item)) {
-        Py_ssize_t start, stop, step, slicelength, cur, i;
+        Py_ssize_t start, stop, step, slicelength, i;
+        size_t cur;
         char* source_buf;
         char* result_buf;
         PyObject* result;