nullify freed pointers in list of dump data

src/output.cpp

@@ -832,6 +832,8 @@ void Output::delete_dump(const std::string &id)
     ivar_dump[i-1] = ivar_dump[i];
   }
   ndump--;
+  dump[ndump] = nullptr;
+  var_dump[ndump] = nullptr;
 }
 
 /* ----------------------------------------------------------------------