fix compile error in debug mode with assertion enabled

If I compile in debug mode I get these errors:
In file included from /usr/include/c++/4.8/cassert:43:0,
                 from /home/agebert/src/node-sapnwrfc/src/Function.cc:26:
/home/agebert/src/node-sapnwrfc/src/Function.cc: In static member function 'static v8::Handle<v8::Value> Function::NewInstance(Connection&, const v8::Arguments&)':
/home/agebert/src/node-sapnwrfc/src/Function.cc:65:10: error: 'handle' was not declared in this scope
   assert(handle != nullptr);
          ^
/home/agebert/src/node-sapnwrfc/src/Function.cc: In static member function 'static void Function::EIO_Invoke(uv_work_t*)':
/home/agebert/src/node-sapnwrfc/src/Function.cc:277:17: error: 'class Function::InvocationBaton' has no member named 'connectionHandle'
   assert(baton->connectionHandle != nullptr);