messaging: fix bad return type in string deserializer
messaging: fix bad return type in string deserializer

Found by gcc 6.

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