[SPARK-4033][Examples]Input of the SparkPi  too big causes the emption exception

If input of the SparkPi args is larger than the 25000, the integer 'n' inside the code will be overflow, and may be a negative number.
And it causes  the (0 until n) Seq as an empty seq, then doing the action 'reduce'  will throw the UnsupportedOperationException("empty collection").

The max size of the input of sc.parallelize is Int.MaxValue - 1, not the Int.MaxValue.
@srowen when the value n is larger than the Int.MaxValue, it's better to set just a small value for slices.But user inputting a big slices may just wish that the SparkPi can run for more time.So I just  limit the size of Seqs and do not modify the parallelism of the app.