Fix for sub_Slice when slice->start is <1

Created by: willmclaren

If a slice has start<1, then it is valid to request an end that is > slice->end as long as it is <=slice->length, since sub_Slice args are relative to the original slice.

Please do correct me if I'm wrong here! I am thoroughly confused by the concept of slices with start<1 to begin with :-)