Wrong command line argument handling in sample without_cpp11_thread_tests

[HerbertKoelman]

Thread number argument is at position 1 and not 0.

   int number_of_threads = 2;
    if ( argc > 0 ){
       number_of_threads = atoi(argv[1]);
    }

Therefore we must test that argc is exactly equal to 2 (argument 1 is the program name and is always there)