fixup! gh-111089: Add cache to PyUnicode_AsUTF8() for embedded NUL

Fix unicode_subtype_new

Author: vstinner

Objects/unicodeobject.c

@@ -14668,6 +14668,7 @@ unicode_subtype_new(PyTypeObject *type, PyObject *unicode)
     _PyUnicode_STATE(self).compact = 0;
     _PyUnicode_STATE(self).ascii = _PyUnicode_STATE(unicode).ascii;
     _PyUnicode_STATE(self).statically_allocated = 0;
+    _PyUnicode_STATE(self).embed_null = 2;
     _PyUnicode_UTF8_LENGTH(self) = 0;
     _PyUnicode_UTF8(self) = NULL;
     _PyUnicode_DATA_ANY(self) = NULL;