Use index based string access for substr with length of 1

apps/files_external/lib/Lib/Storage/OwnCloud.php

apps/files_external/lib/Lib/Storage/SFTP.php

apps/files_external/lib/Lib/Storage/SMB.php

lib/private/App/AppStore/Version/VersionParser.php

lib/private/Archive/TAR.php

lib/private/Avatar.php

lib/private/Files/Filesystem.php

lib/private/Files/Storage/DAV.php

lib/private/Files/View.php

lib/private/Installer.php

lib/private/Settings/Personal/PersonalInfo.php

lib/private/legacy/image.php