Fixed possible page number issue updating ToC in non-PDF conversions.

Fixes issue described at:
   * http://www.oooforum.org/forum/viewtopic.phtml?t=7826
   * https://issues.apache.org/ooo/show_bug.cgi?id=29165

Bad page numbers can be assigned under certain circumstances to ToC entries when an `XDocumentIndex.update()` operation is executed. This only happens in some scenarios and with specific documents, and seems to be related to some document loading issue in UNO.

To-PDF conversion is not affected, but to-ODT is (including ODT-to-ODT, which we use for updating a ToC coming from a merged document template).