use more std::unique_ptr in ScUndoDraw

Change-Id: I4216efd74f279a0997754b75b62b2dedce09511e
Reviewed-on: https://gerrit.libreoffice.org/61434
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

sc/source/ui/inc/undodraw.hxx

@@ -37,7 +37,7 @@ public:
                             ScUndoDraw( std::unique_ptr<SfxUndoAction> pUndo, ScDocShell* pDocSh );
     virtual                 ~ScUndoDraw() override;
 
-    SfxUndoAction*          ReleaseDrawUndo()   { return pDrawUndo.release(); }
+    std::unique_ptr<SfxUndoAction> ReleaseDrawUndo()   { return std::move(pDrawUndo); }
 
     virtual void            Undo() override;
     virtual void            Redo() override;

sc/source/ui/undo/undobase.cxx

@@ -77,7 +77,7 @@ bool ScSimpleUndo::Merge( SfxUndoAction *pNextAction )
         // ScUndoDraw is later deleted by the UndoManager
 
         ScUndoDraw* pCalcUndo = static_cast<ScUndoDraw*>(pNextAction);
-        pDetectiveUndo.reset( pCalcUndo->ReleaseDrawUndo() );
+        pDetectiveUndo = pCalcUndo->ReleaseDrawUndo();
         return true;
     }