BibIndex: fix double-stemming upon indexing

• In get_words_from_phrase(), certain phrases could have resulted in returning double-stemmed terms, thus causing these terms not to be correctly indexed.

• In order to fully apply the fix one have also to run
  
  $ bibindex --reindex
  
  on any index that is configured to use stemming.

Co-authored-by: Ludmila Marian <ludmila.marian@gmail.com>