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

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.

Tools/peg_generator/pegen/c_generator.py

@@ -88,7 +88,7 @@ class FunctionCall:
         if self.arguments:
             parts.append(f"({', '.join(map(str, self.arguments))})")
         if self.force_true:
-            parts.append(", 1")
+            parts.append(", !p->error_indicator")
         if self.assigned_variable:
             if self.assigned_variable_type:
                 parts = [