fix invalid string access in OUString::copy

Change-Id: Idccec35f2838b54a7915aa4a0aeb229c69bae74d
only write new element and attributes in ODF 1.2 extended

Change-Id: I038f200f500ef1c6a0815b6332fb17bb2f62282a
read elements/attribs written to extension namespace

Related to validation errors with fdo#31296

Change-Id: I1b76e05a9996b1123db164629475a3f7b07dda69