ScInterpreter::GetString can return reference to shared string instance

...ScInterpreter::aTempStr member.  Regression introduced with
a11be8a8 "use size_t where possible and fix some
more places" causing JunitTest_sc_unoapi's
sc.ScDDELinkObj::com::sun::star::util::XRefreshable to fail.

Change-Id: I50227b004e37f68c5040d4ace90a79300f711be8

sc/source/core/tool/interpr2.cxx

@@ -2255,9 +2255,9 @@ void ScInterpreter::ScDde()
         sal_uInt8 nMode = SC_DDE_DEFAULT;
         if (nParamCount == 4)
             nMode = (sal_uInt8) ::rtl::math::approxFloor(GetDouble());
-        const OUString& aItem  = GetString();
-        const OUString& aTopic = GetString();
-        const OUString& aAppl  = GetString();
+        OUString aItem  = GetString();
+        OUString aTopic = GetString();
+        OUString aAppl  = GetString();
 
         if (nMode > SC_DDE_TEXT)
             nMode = SC_DDE_DEFAULT;