fix buffer overflow in string
fix buffer overflow in string

Summary: Stack trace of the failure triggered by asan: P148008320

Reviewed By: rgiese

Differential Revision: D24715355

fbshipit-source-id: 96e702704fe4796cd329cbba79915a31bbae5723

rsocket/rsocket-cpp,rsocket/rsocket-cpp,ReactiveSocket/reactivesocket-cpp,rsocket/rsocket-cpp,rsocket/rsocket-cpp,ReactiveSocket/reactivesocket-cpp,ReactiveSocket/reactivesocket-cpp