thesaurus function breaks if a key has no feature match  #115

mydict <- dictionary(list(nonExistent = c("XXX", "YYY")))
mydfm <- dfm(inaugCorpus, dictionary = mydict)
Creating a dfm from a corpus ...
  ... lowercasing
  ... tokenizing
  ... indexing documents: 57 documents
  ... indexing features: 9,215 feature types
  ... applying a dictionary consisting of 1 key
  ... created a 57 x 1 sparse dfm
  ... complete. 
Elapsed time: 0.115 seconds.
topfeatures(mydfm)
nonExistent 
         0 
mydfm <- dfm(inaugCorpus, thesaurus = mydict)
Creating a dfm from a corpus ...
  ... lowercasing
  ... tokenizing
  ... indexing documents: 57 documents
  ... indexing features: 9,215 feature types
  ... applying a dictionary consisting of 1 key
Show Traceback

Rerun with Debug
Error in -keyIndex : invalid argument to unary operator