[ios] Fix substring range length calculation for unicode emoji.
[ios] Fix substring range length calculation for unicode emoji.

msand/react-native-svg,magicismight/react-native-art-svg,magicismight/react-native-art-svg,msand/react-native-svg,magicismight/react-native-svg,react-native-community/react-native-svg,react-native-community/react-native-svg,magicismight/react-native-svg,magicismight/react-native-svg,msand/react-native-svg,magicismight/react-native-svg,react-native-community/react-native-svg,msand/react-native-svg,react-native-community/react-native-svg,magicismight/react-native-svg,react-native-community/react-native-svg