Removed redundant call to `std::string::c_str()` in stout.
Removed redundant call to `std::string::c_str()` in stout.

Spotted using the "readability-redundant-string-cstr" clang-tidy check.

Review: https://reviews.apache.org/r/58037

jpeach/mesos,reneploetz/mesos,asamerh4/mesos,craimbert/mesos,verizonlabs/mesos,verizonlabs/mesos,asamerh4/mesos,gsantovena/mesos,dforsyth/mesos,asamerh4/mesos,asamerh4/mesos,jpeach/mesos,Gilbert88/mesos,dforsyth/mesos,zmalik/mesos,dforsyth/mesos,abudnik/mesos,reneploetz/mesos,zmalik/mesos,andschwa/mesos,kaysoky/mesos,Gilbert88/mesos,abudnik/mesos,chhsia0/mesos,craimbert/mesos,verizonlabs/mesos,Gilbert88/mesos,asamerh4/mesos,asamerh4/mesos,zmalik/mesos,chhsia0/mesos,jpeach/mesos,chhsia0/mesos,kaysoky/mesos,zmalik/mesos,dforsyth/mesos,gsantovena/mesos,dforsyth/mesos,chhsia0/mesos,gsantovena/mesos,kaysoky/mesos,neilconway/mesos,jpeach/mesos,Gilbert88/mesos,reneploetz/mesos,jpeach/mesos,kaysoky/mesos,abudnik/mesos,craimbert/mesos,neilconway/mesos,andschwa/mesos,abudnik/mesos,craimbert/mesos,kaysoky/mesos,kaysoky/mesos,andschwa/mesos,Gilbert88/mesos,craimbert/mesos,reneploetz/mesos,kaysoky/mesos,zmalik/mesos,andschwa/mesos,verizonlabs/mesos,andschwa/mesos,gsantovena/mesos,dforsyth/mesos,zmalik/mesos,reneploetz/mesos,abudnik/mesos,verizonlabs/mesos,andschwa/mesos,craimbert/mesos,jpeach/mesos,gsantovena/mesos,gsantovena/mesos,craimbert/mesos,jpeach/mesos,abudnik/mesos,chhsia0/mesos,abudnik/mesos,chhsia0/mesos,dforsyth/mesos,zmalik/mesos,neilconway/mesos,neilconway/mesos,neilconway/mesos,Gilbert88/mesos,neilconway/mesos,andschwa/mesos,neilconway/mesos,Gilbert88/mesos,reneploetz/mesos,verizonlabs/mesos,chhsia0/mesos,verizonlabs/mesos,gsantovena/mesos,reneploetz/mesos,asamerh4/mesos