[3.9] bpo-45494: Fix parser crash when reporting errors involving invalid continuatio...

[3.9] bpo-45494: Fix parser crash when reporting errors involving invalid continuation characters (GH-28993) (#29071)

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>
NOTE: unlike the cherry-picked original, this commit points at a crazy location
due to a bug in the tokenizer that required a big refactor in 3.10 to fix.
We are leaving as-is for 3.9.