Fix crash when calling substring() on a string containing emoji. (facebook#23609)

Summary:
Fixes facebook#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: facebook#23609

Differential Revision: D14198159

Pulled By: mdvacca

fbshipit-source-id: 71060b1b99ddab89793c98c09f99ec9974479e62

Author: ericlewis

RNTester/RNTesterIntegrationTests/ReferenceImages/RNTester-js-RNTesterApp.ios/testTextExample_1-iOS12@2x.png

@@ -615,11 +615,13 @@ class TextExample extends React.Component<{}> {
             Works with other text styles
           </Text>
         </RNTesterBlock>
+        <RNTesterBlock title="Substring Emoji (should only see 'test')">
+          <Text>{'test🙃'.substring(0, 5)}</Text>
+        </RNTesterBlock>
       </RNTesterPage>
     );
   }
 }
-
 const styles = StyleSheet.create({
   backgroundColorText: {
     left: 5,

RNTester/js/TextExample.android.js

@@ -428,6 +428,12 @@ exports.examples = [
       );
     },
   },
+  {
+    title: "Substring Emoji (should only see 'test')",
+    render: function() {
+      return <Text>{'test🙃'.substring(0, 5)}</Text>;
+    },
+  },
   {
     title: 'Text metrics',
     render: function() {

RNTester/js/TextExample.ios.js

@@ -238,14 +238,10 @@ class JSCRuntime : public jsi::Runtime {
 // JSStringRef utilities
 namespace {
 std::string JSStringToSTLString(JSStringRef str) {
-  std::string result;
   size_t maxBytes = JSStringGetMaximumUTF8CStringSize(str);
-  result.resize(maxBytes);
-  size_t bytesWritten = JSStringGetUTF8CString(str, &result[0], maxBytes);
-  // JSStringGetUTF8CString writes the null terminator, so we want to resize
-  // to `bytesWritten - 1` so that `result` has the correct length.
-  result.resize(bytesWritten - 1);
-  return result;
+  std::vector<char> buffer(maxBytes);
+  JSStringGetUTF8CString(str, buffer.data(), maxBytes);
+  return std::string(buffer.data());
 }
 
 JSStringRef getLengthString() {