Fix check for empty stack

Regression introduced with ecfcc54543 "Replace
Stack with std::stack< std::pair< XclObj*, XclEscherHostAppData* > >"

Change-Id: Ib4776a2df3c2f9648f2e1af70c7d20485a7ff888

sc/source/filter/xcl97/xcl97esc.cxx

@@ -109,7 +109,7 @@ XclEscherEx::XclEscherEx( const XclExpRoot& rRoot, XclExpObjectManager& rObjMgr,
 
 XclEscherEx::~XclEscherEx()
 {
-    OSL_ENSURE( !aStack.empty(), "~XclEscherEx: stack not empty" );
+    OSL_ENSURE( aStack.empty(), "~XclEscherEx: stack not empty" );
     DeleteCurrAppData();
     delete pTheClientData;
 }