Share exception bug

NULL Ref stays on the stack after plugin throws an exception. The m_refs.pop() should be moved from:

if (data_ref && !ref()) {
    m_refs.pop();
    throw Error{PDI_ERR_RIGHT, "Unable to grant requested rights"};
}

to:

catch (...) {
    // on error, do not free the data as would be done automatically otherwise
    r.release();
    assert((!metadata() || !m_refs.empty()) && "metadata descriptors should always keep a placeholder");
    throw;
}