fix comment

Author: shadchin

Objects/unicodeobject.c

@@ -1430,10 +1430,13 @@ _copy_characters(PyObject *to, Py_ssize_t to_start,
     assert(from_start + how_many <= PyUnicode_GET_LENGTH(from));
 
     assert(to == NULL || PyUnicode_Check(to));
-    assert(how_many == 0 || (to_start + how_many <= PyUnicode_GET_LENGTH(to)));
 
-    if (how_many == 0)
+    if (how_many == 0) {
         return 0;
+    }
+
+    assert(to != NULL);
+    assert(to_start + how_many <= PyUnicode_GET_LENGTH(to));
 
     from_kind = PyUnicode_KIND(from);
     from_data = PyUnicode_DATA(from);