Fix bug in gapped kmer

[thomasopsomer]

Hello @jmschrei,

I had issues in the function that computes scores for the gapped kmers: _extract_gkmers in _seqlet_to_gkmers. It seems that if there is less gapped kmers extracted than the parameter max_entries there is a bug. It's just a quick fix that ensure we don't have size issues but I didn't really think this a lot :)