Ensure the str member of the tokenizer is always initialised (GH-29681)

Parser/pegen_errors.c

@@ -245,7 +245,7 @@ get_error_line_from_tokenizer_buffers(Parser *p, Py_ssize_t lineno)
      * (multi-line) statement are stored in p->tok->interactive_src_start.
      * If not, we're parsing from a string, which means that the whole source
      * is stored in p->tok->str. */
-    assert(p->tok->fp == NULL || p->tok->fp == stdin);
+    assert((p->tok->fp == NULL && p->tok->str != NULL) || p->tok->fp == stdin);
 
     char *cur_line = p->tok->fp_interactive ? p->tok->interactive_src_start : p->tok->str;
     assert(cur_line != NULL);