sstring: fix conversion to std::string with NUL bytes
sstring: fix conversion to std::string with NUL bytes

sstring's std::string conversion uses c_str() to construct the value,
but the conversion is broken if the value contains NUL - both sstring and
std::string can contain NULs, but C strings use them as a terminator.

Fix by using the size+length std::string constructor.

shaunstanislaus/scylla,kjniemi/scylla,stamhe/scylla,acbellini/seastar,dreamsxin/seastar,dwdm/scylla,scylladb/scylla,sjperkins/seastar,glommer/scylla,duarten/scylla,bowlofstew/scylla,slivne/seastar,respu/scylla,wildinto/scylla,leejir/seastar,eklitzke/scylla,linearregression/seastar,senseb/scylla,gwicke/scylla,scylladb/scylla,raphaelsc/scylla,raphaelsc/seastar,gwicke/scylla,scylladb/scylla-seastar,ducthangho/imdb,aruanruan/scylla,duarten/scylla,avikivity/seastar,bowlofstew/scylla,printedheart/seastar,avikivity/scylla,justintung/scylla,kjniemi/scylla,joerg84/seastar,jonathanleang/seastar,tempbottle/seastar,printedheart/seastar,kangkot/scylla,kjniemi/seastar,respu/scylla,tempbottle/scylla,joerg84/seastar,rluta/scylla,rentongzhang/scylla,bowlofstew/scylla,norcimo5/seastar,shyamalschandra/seastar,rluta/scylla,tempbottle/scylla,rentongzhang/scylla,sjperkins/seastar,victorbriz/scylla,scylladb/seastar,phonkee/scylla,justintung/scylla,ducthangho/imdb,avikivity/scylla,justintung/scylla,chunshengster/seastar,bzero/seastar,wildinto/seastar,dwdm/scylla,asias/scylla,linearregression/seastar,raphaelsc/scylla,wildinto/seastar,sjperkins/seastar,aruanruan/scylla,acbellini/seastar,flashbuckets/seastar,kjniemi/scylla,acbellini/scylla,joerg84/seastar,leejir/seastar,victorbriz/scylla,avikivity/scylla,kangkot/scylla,xtao/seastar,guiquanz/scylla,tempbottle/scylla,norcimo5/seastar,capturePointer/scylla,dwdm/seastar,tempbottle/seastar,acbellini/seastar,stamhe/seastar,flashbuckets/seastar,glommer/scylla,mixja/seastar,shyamalschandra/seastar,asias/scylla,raphaelsc/seastar,wildinto/seastar,linearregression/scylla,cloudius-systems/seastar,hongliangzhao/seastar,acbellini/scylla,capturePointer/scylla,cloudius-systems/seastar,syuu1228/seastar,senseb/scylla,gwicke/scylla,capturePointer/scylla,stamhe/scylla,slivne/seastar,stamhe/scylla,rluta/scylla,syuu1228/seastar,kjniemi/seastar,wildinto/scylla,jonathanleang/seastar,chunshengster/seastar,kangkot/scylla,glommer/scylla,slivne/seastar,mixja/seastar,raphaelsc/scylla,chunshengster/seastar,dwdm/seastar,jonathanleang/seastar,linearregression/scylla,scylladb/seastar,koolhazz/seastar,eklitzke/scylla,mixja/seastar,avikivity/seastar,koolhazz/seastar,shyamalschandra/seastar,syuu1228/seastar,anzihenry/seastar,dwdm/seastar,flashbuckets/seastar,scylladb/scylla-seastar,duarten/scylla,hongliangzhao/seastar,bzero/seastar,kjniemi/seastar,bzero/seastar,scylladb/seastar,aruanruan/scylla,linearregression/scylla,hongliangzhao/seastar,tempbottle/seastar,avikivity/seastar,senseb/scylla,dreamsxin/seastar,guiquanz/scylla,scylladb/scylla,koolhazz/seastar,bowlofstew/seastar,scylladb/scylla-seastar,bowlofstew/seastar,dwdm/scylla,stamhe/seastar,ducthangho/imdb,phonkee/scylla,norcimo5/seastar,dreamsxin/seastar,shaunstanislaus/scylla,rentongzhang/scylla,respu/scylla,raphaelsc/seastar,acbellini/scylla,eklitzke/scylla,guiquanz/scylla,xtao/seastar,anzihenry/seastar,wildinto/scylla,anzihenry/seastar,shaunstanislaus/scylla,bowlofstew/seastar,xtao/seastar,scylladb/scylla,asias/scylla,printedheart/seastar,phonkee/scylla,stamhe/seastar,linearregression/seastar,leejir/seastar,victorbriz/scylla,cloudius-systems/seastar