Fixes IndexError when converting sequence to matrix
Fixes IndexError when converting sequence to matrix

Calling sequences_to_matrix results in an IndexError when nb_words = None. This is caused by a 1-indexed word_index, since sequences_to_matrix expects 0-indexing. Converts word_index to 0-based indexing.
zxsted/keras,jasonyaw/keras,why11002526/keras,amy12xx/keras,3dconv/keras,xiaoda99/keras,EderSantana/keras,meanmee/keras,stephenbalaban/keras,ekamioka/keras,keras-team/keras,pjadzinsky/keras,DeepGnosis/keras,iScienceLuvr/keras,saurav111/keras,dxj19831029/keras,iamtrask/keras,abayowbo/keras,nzer0/keras,Yingmin-Li/keras,jimgoo/keras,zxytim/keras,chenych11/keras,vseledkin/keras,bboalimoe/keras,florentchandelier/keras,johmathe/keras,rudaoshi/keras,keskarnitish/keras,Aureliu/keras,nehz/keras,ashhher3/keras,harshhemani/keras,zhmz90/keras,nt/keras,Smerity/keras,danielforsyth/keras,fmacias64/keras,yingzha/keras,relh/keras,untom/keras,gamer13/keras,LIBOTAO/keras,MagicSen/keras,kemaswill/keras,eulerreich/keras,pthaike/keras,ledbetdr/keras,bottler/keras,wxs/keras,brainwater/keras,jalexvig/keras,llcao/keras,cvfish/keras,keras-team/keras,printedheart/keras,sjuvekar/keras,imcomking/Convolutional-GRU-keras-extension-,nebw/keras,kuza55/keras,DLlearn/keras,dolaameng/keras,daviddiazvico/keras,xurantju/keras,JasonTam/keras,rodrigob/keras,hhaoyan/keras