N50, N70, ..., N90 bug fix

After the for loop the indices point to the next scaffold/contigs.
Resetting only the sequence_index was a severe bug.
Resetting both the index and sum should be ok. 
But, it should be just redundant recalculation of the same thing.

Here is a test fasta file to check the behavior :)

>a
acgtacgtacgtaagNNNNNNNNNNNNNNNNNagaatatcaggatatagagatca
>b
acgatatacagata