Boundary check in PickNegLogSoftmax (#1026)
Boundary check in PickNegLogSoftmax (#1026)

* Boundary check in PickNegLogSoftmax

* Bug fix


Former-commit-id: 3753675d419feb44b3194a9eaf9bb812b22e0ecf
chunyang-wen/dynet,danielhers/dynet,chunyang-wen/dynet,shuheik/dynet,danielhers/dynet,clab/dynet,shuheik/dynet,clab/cnn,clab/cnn,clab/dynet,clab/dynet,shuheik/dynet,danielhers/dynet,danielhers/dynet,clab/dynet,clab/dynet,clab/cnn,xunzhang/dynet,shuheik/dynet,chunyang-wen/dynet,danielhers/dynet,xunzhang/dynet,shuheik/dynet,danielhers/dynet,clab/dynet,danielhers/dynet,shuheik/dynet,xunzhang/dynet,clab/dynet,chunyang-wen/dynet,shuheik/dynet,xunzhang/dynet