Fix crash when calling substring() on a string containing emoji.

[ericlewis]

Summary

Fixes #23459. It is not legal to write the character array of a std::string, and can result in undefined behavior.

Changelog

[General] [Fixed] - Crash when substring intersects with emoji

Test Plan

Updated RNTester TextExample.ios.js & TextExample.android.js with a section containing the below:

<Text>{'test🙃'.substring(0, 5)}</Text>`

Also updated the test snapshot for iOS. So long as it doesn't crash tho, the test technically is passed.

[facebook-github-bot]

@mdvacca has imported this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[facebook-github-bot]

@mdvacca has imported this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[ericlewis]

@cpojer @hramos anything I need to do to get unstuck?

[hramos]

@ericlewis I followed up on the imported diff and it looks like David is seeking additional review from some fellow team members. No action needed on your part for now. Thanks for the PR!

[mdvacca]

@ericlewis the PR is approved, I tried to land this but got conflicts on the png. can you rebase it? I will land it after. Thanks

[ericlewis]

@mdvacca done! (tho I seem to have messed up the history a bit 🤔)

[cpojer]

@ericlewis yeah it seems like the PR was messed up. Could you rebase on top of master and force push to your branch with only the changes you made?

[ericlewis]

@cpojer fixed!

[facebook-github-bot]

@mdvacca is landing this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[react-native-bot]

This pull request was successfully merged by @ericlewis in 58c3a4c.

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