CP-SAT(java): Log handler crash when having NumSearchWorkers > 1

[Mizux]

When using more than one worker, the log handler callback will crash the jvm

To reprodue

final CpSolver solver = new CpSolver();
StringBuilder logBuilder = new StringBuilder();
Consumer<String> appendToLog = (String message) -> {
  System.out.println(
  "Current Thread Name:" + Thread.currentThread().getName()
  + " Id:" + Thread.currentThread().getId()
  + " msg:" + message
  );
  logBuilder.append(message).append('\n');
};
solver.setLogCallback(appendToLog);
solver.getParameters()
  .setLogToStdout(false)
  .setLogSearchProgress(true)
  .setNumSearchWorkers(12);
CpSolverStatus status = solver.solve(model);