Bugfix -- Crashed on unknown words in evaluation.
Bugfix -- Crashed on unknown words in evaluation.


Former-commit-id: e00c183582baa340147efc18a97292929cc46f7a
danielhers/dynet,clab/cnn,xunzhang/dynet,shuheik/dynet,chunyang-wen/dynet,clab/dynet,danielhers/dynet,shuheik/dynet,danielhers/dynet,clab/dynet,shuheik/dynet,danielhers/dynet,chunyang-wen/dynet,clab/cnn,clab/dynet,clab/dynet,clab/dynet,shuheik/dynet,danielhers/dynet,danielhers/dynet,chunyang-wen/dynet,clab/dynet,xunzhang/dynet,shuheik/dynet,shuheik/dynet,xunzhang/dynet,chunyang-wen/dynet,clab/cnn,clab/dynet,shuheik/dynet,danielhers/dynet,xunzhang/dynet