[3.10] Ensure the str member of the tokenizer is always initialised (GH-29681). (GH-29683)

(cherry picked from commit 4f006a789a)

Co-authored-by: Pablo Galindo Salgado <Pablogsal@gmail.com>

Parser/tokenizer.c

@@ -86,7 +86,7 @@ tok_new(void)
     tok->async_def_indent = 0;
     tok->async_def_nl = 0;
     tok->interactive_underflow = IUNDERFLOW_NORMAL;
-
+    tok->str = NULL;
     return tok;
 }