TableRef: clone inner reference RPN token only if it exists

that author was stupid ...

Change-Id: Ib06e88be52e70dee72ecb00590adce696eb347c0
(cherry picked from commit 1f832584)

sc/source/core/tool/token.cxx

@@ -916,7 +916,7 @@ ScTableRefToken::ScTableRefToken( sal_uInt16 nIndex, ScTableRefToken::Item eItem
 
 ScTableRefToken::ScTableRefToken( const ScTableRefToken& r ) :
     FormulaToken(r),
-    mxAreaRefRPN(r.mxAreaRefRPN->Clone()),
+    mxAreaRefRPN( r.mxAreaRefRPN ?  r.mxAreaRefRPN->Clone() : nullptr),
     mnIndex(r.mnIndex),
     meItem(r.meItem)
 {