[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.h

@@ -71,7 +71,7 @@ struct tok_state {
     PyObject *decoding_readline; /* open(...).readline */
     PyObject *decoding_buffer;
     const char* enc;        /* Encoding for the current str. */
-    char* str;
+    char* str;          /* Source string being tokenized (if tokenizing from a string)*/
     char* input;       /* Tokenizer's newline translated copy of the string. */
 
     int type_comments;      /* Whether to look for type comments */