keys: Fix prefixable_full_tuple::is_prefixed_by()
keys: Fix prefixable_full_tuple::is_prefixed_by()

Spotted during code review.

respu/scylla,duarten/scylla,gwicke/scylla,kjniemi/scylla,stamhe/scylla,senseb/scylla,scylladb/scylla,respu/scylla,stamhe/scylla,duarten/scylla,asias/scylla,scylladb/scylla,victorbriz/scylla,linearregression/scylla,wildinto/scylla,eklitzke/scylla,victorbriz/scylla,avikivity/scylla,aruanruan/scylla,justintung/scylla,capturePointer/scylla,eklitzke/scylla,glommer/scylla,wildinto/scylla,respu/scylla,victorbriz/scylla,kangkot/scylla,tempbottle/scylla,guiquanz/scylla,kangkot/scylla,aruanruan/scylla,shaunstanislaus/scylla,shaunstanislaus/scylla,acbellini/scylla,aruanruan/scylla,rentongzhang/scylla,acbellini/scylla,bowlofstew/scylla,dwdm/scylla,acbellini/scylla,asias/scylla,bowlofstew/scylla,capturePointer/scylla,raphaelsc/scylla,tempbottle/scylla,linearregression/scylla,guiquanz/scylla,glommer/scylla,avikivity/scylla,scylladb/scylla,gwicke/scylla,stamhe/scylla,avikivity/scylla,phonkee/scylla,rluta/scylla,kangkot/scylla,senseb/scylla,eklitzke/scylla,rentongzhang/scylla,gwicke/scylla,dwdm/scylla,glommer/scylla,tempbottle/scylla,phonkee/scylla,duarten/scylla,rluta/scylla,scylladb/scylla,dwdm/scylla,asias/scylla,justintung/scylla,rentongzhang/scylla,kjniemi/scylla,justintung/scylla,rluta/scylla,shaunstanislaus/scylla,wildinto/scylla,raphaelsc/scylla,guiquanz/scylla,senseb/scylla,phonkee/scylla,raphaelsc/scylla,bowlofstew/scylla,capturePointer/scylla,linearregression/scylla,kjniemi/scylla