out-of-bounds arg mapping access, this was always wrong but never hit it seems

Change-Id: I15843b320ac8ddcefce8094da7aa7c6b81b42e4f
(cherry picked from commit 5b404cc2)
Reviewed-on: https://gerrit.libreoffice.org/20773Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>

formula/source/ui/dlg/parawin.cxx

@@ -118,7 +118,7 @@ void ParaWin::UpdateArgDesc( sal_uInt16 nArg )
 
         if ( nArgs < VAR_ARGS )
         {
-            sal_uInt16 nRealArg = (aVisibleArgMapping.size() < nArg) ? aVisibleArgMapping[nArg] : nArg;
+            sal_uInt16 nRealArg = (nArg < aVisibleArgMapping.size()) ? aVisibleArgMapping[nArg] : nArg;
             aArgDesc  = pFuncDesc->getParameterDescription(nRealArg);
             aArgName  = pFuncDesc->getParameterName(nRealArg);
             aArgName += " ";