Summary:
Fixes#23459. It is not legal to write the character array of a std::string, and can result in undefined behavior.
[General] [Fixed] - Crash when substring intersects with emoji
Pull Request resolved: https://github.com/facebook/react-native/pull/23609
Differential Revision: D14198159
Pulled By: mdvacca
fbshipit-source-id: 71060b1b99ddab89793c98c09f99ec9974479e62