use boost::optional for OUString instead of std::unique_ptr

Change-Id: I4539380956dad232f3ce92498b8f58a205fe00b1
Reviewed-on: https://gerrit.libreoffice.org/58196
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

filter/source/msfilter/svdfppt.cxx

@@ -6774,7 +6774,7 @@ PPTTextObj::PPTTextObj( SvStream& rIn, SdrPowerPointImport& rSdrPowerPointImport
                                             if (rPersistEntry.xHeaderFooterEntry->nAtom & 0x20000)    // auto date time
                                                 xEntry->SetDateTime(rPersistEntry.xHeaderFooterEntry->nAtom & 0xff);
                                             else
-                                                xEntry->xString.reset(new OUString(rPersistEntry.xHeaderFooterEntry->pPlaceholder[nVal]));
+                                                xEntry->xString = rPersistEntry.xHeaderFooterEntry->pPlaceholder[nVal];
                                         }
                                     }
                                     break;
@@ -6826,7 +6826,7 @@ PPTTextObj::PPTTextObj( SvStream& rIn, SdrPowerPointImport& rSdrPowerPointImport
                                                             else if (!n)
                                                             {
                                                                 // End of format string
-                                                                xEntry->xString.reset(new OUString( aStr ));
+                                                                xEntry->xString = aStr;
                                                                 break;
                                                             }
                                                             else if (!inquote)

include/filter/msfilter/svdfppt.hxx

@@ -45,6 +45,7 @@
 #include <tools/solar.h>
 #include <vcl/graph.hxx>
 #include <salhelper/simplereferenceobject.hxx>
+#include <boost/optional.hpp>
 
 namespace boost {
     template <class T> class optional;
@@ -495,7 +496,7 @@ struct MSFILTER_DLLPUBLIC PPTFieldEntry
     sal_uInt16          nTextRangeEnd;
     std::unique_ptr<SvxFieldItem> xField1;
     std::unique_ptr<SvxFieldItem> xField2;
-    std::unique_ptr<OUString> xString;
+    boost::optional<OUString> xString;
 
     PPTFieldEntry()
         : nPos(0)