[3.10] bpo-45494: Fix parser crash when reporting errors involving invalid continuation characters (GH-28993) (GH-29070)

There are two errors that this commit fixes:

* The parser was not correctly computing the offset and the string
  source for E_LINECONT errors due to the incorrect usage of strtok().
* The parser was not correctly unwinding the call stack when a tokenizer
  exception happened in rules involving optionals ('?', [...]) as we
  always make them return valid results by using the comma operator. We
  need to check first if we don't have an error before continuing..
(cherry picked from commit a106343f632a99c8ebb0136fa140cf189b4a6a57)

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