sstables: don't pass temporary string to regex
sstables: don't pass temporary string to regex

Since the regex match returns views into that string, it must not be
a temporary. gcc 5.1's libstdc++ won't accept it, either.

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