Do not overwrite data with same value

After emplace_back there are at least 2 elements, and GetPrevPos
would point to previous back().
Commit 5e913234 removed
a possible modification of such data, so this operation is now
pointless.

Change-Id: Ia5bec5e9b16e9dc9f34c7cf1dbb8e3f17510d83b

svx/source/svdraw/svddrag.cxx

@@ -72,9 +72,7 @@ void SdrDragStat::NextMove(const Point& rPnt)
 
 void SdrDragStat::NextPoint()
 {
-    Point aPnt(mvPnts.back());
     mvPnts.emplace_back(aRealNow);
-    mvPnts[GetPrevPos()] = aPnt;
 }
 
 void SdrDragStat::PrevPoint()