each time when it is converted, the string will be set once, but in the
setString logic, the text is inserted instead of set

Repeated conversion is unnecessary, add a flag to avoid repeated conversion

(cherry picked from commit d39da407)

Conflicts:
	sc/source/filter/oox/richstring.cxx

Change-Id: I3ae08d59050ce84fb6ca0f2a82acdd27e71ba52b