Fix misuse of PyUnicode_GET_SIZE, use PyUnicode_GET_LENGTH instead
diff --git a/Modules/_json.c b/Modules/_json.c
index e49d1b2..5a79294 100644
--- a/Modules/_json.c
+++ b/Modules/_json.c
@@ -837,7 +837,7 @@
 
     /* rval = parse_constant(constant) */
     rval = PyObject_CallFunctionObjArgs(s->parse_constant, cstr, NULL);
-    idx += PyUnicode_GET_SIZE(cstr);
+    idx += PyUnicode_GET_LENGTH(cstr);
     Py_DECREF(cstr);
     *next_idx_ptr = idx;
     return rval;