Heap buffer overflow in OOXML text box element import

LibreOffice can import documents in the OOXML format (DOCX). A heap buffer overflow existed when replaying deferred parser events for a text box element. A handler object was assumed to be of one type and written to at that type's field layout, but it could be a smaller object, so the write landed past the end of the allocation. In fixed versions the type is checked before the write.