#3367 from Kristjan Valur Jonsson:

If a PyTokenizer_FromString() is called with an empty string, the
tokenizer's line_start member never gets initialized.  Later, it is
compared with the token pointer 'a' in parsetok.c:193 and that behavior
can result in undefined behavior.

Parser/tokenizer.c

@@ -1117,7 +1117,7 @@ tok_get(register struct tok_state *tok, char **p_start, char **p_end)
 	register int c;
 	int blankline;
 
-	*p_start = *p_end = NULL;
+	tok->line_start = *p_start = *p_end = NULL;
   nextline:
 	tok->start = NULL;
 	blankline = 0;