fix segfault when op is unset (#15)
fix segfault when op is unset (#15)


imai-lm/nnvm,imai-lm/nnvm,piiswrong/nnvm,piiswrong/nnvm,ZihengJiang/nnvm,imai-lm/nnvm,ZihengJiang/nnvm,ZihengJiang/nnvm,piiswrong/nnvm