Resolves: #i124928# rich text portion could be converted several times...

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 d39da40700ceab57e9f80bd9851598e1d455f121)

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

Change-Id: I3ae08d59050ce84fb6ca0f2a82acdd27e71ba52b