SdrMarkList: fix STL conversion

regression from aa11e7ba:
Container::GetObject returned NULL if the index is out of bounds,
and callers seem to rely on that.

Change-Id: I4c9f5249514efad0d516b6ffd6f04dadb019052f

svx/inc/svx/svdmark.hxx

@@ -226,11 +226,7 @@ public:
         return maList.size();
     }
 
-    SdrMark* GetMark(sal_uLong nNum) const
-    {
-        return maList[nNum];
-    }
-
+    SdrMark* GetMark(sal_uLong nNum) const;
     sal_uLong FindObject(const SdrObject* pObj) const;
     void InsertEntry(const SdrMark& rMark, sal_Bool bChkSort = sal_True);
     void DeleteMark(sal_uLong nNum);

svx/source/svdraw/svdmark.cxx

@@ -356,6 +356,11 @@ void SdrMarkList::operator=(const SdrMarkList& rLst)
     mbSorted = rLst.mbSorted;
 }
 
+SdrMark* GetMark(sal_uLong nNum) const
+{
+    return (nNum < maList.size()) ? maList[nNum] : NULL;
+}
+
 sal_uLong SdrMarkList::FindObject(const SdrObject* pObj) const
 {
     // Since relying on OrdNums is not allowed for the selection because objects in the