DOCX import: merging properties of different SDT's are not wanted

When the document starts with a table and both the A1 cell as well as
the document start has an SDT start, then we see both at the same time
when we check for pending SDT's in lcl_utext().

This leads to merging the properties of the two SDT's, which is clearly
not wanted. Fix the problem by clearning the SDT property list when we
see a start of a new property list: that produces a valid document.

Change-Id: I8fbe7bec02beebb26ed99fa7b08bc62225bff50c