gh-111089: PyUnicode_AsUTF8AndSize() sets size on error (#111106)

On error, PyUnicode_AsUTF8AndSize() now sets the size argument to -1,
to avoid undefined value.

Modules/_testcapi/unicode.c

@@ -634,7 +634,7 @@ unicode_asutf8andsize(PyObject *self, PyObject *args)
     NULLABLE(unicode);
     s = PyUnicode_AsUTF8AndSize(unicode, &size);
     if (s == NULL) {
-        assert(size == UNINITIALIZED_SIZE);
+        assert(size == -1);
         return NULL;
     }