Check for null in Hive History

I think there may be some bigger underlying bug here -- that printRowCount() shouldn't be called before the corresponding QueryInfo has been created, or maybe two concurrent queries are using the same queryId, so the QueryInfo is getting deleted prematurely (because two queries are sharing one).  In any case, this makes the printRowCount() method consistent with other methods in this class, that check if the queryinfo is null before doing anything with it.