tokenize.c: Convert checking of last Wordgraph word
tokenize.c: Convert checking of last Wordgraph word

On the last word (a dummy word) the next pointer is NULL.
Convert the checks that uses this to a explicit check, as
this is detail of the current implementation that is going to be changed.

linas/link-grammar,opencog/link-grammar,opencog/link-grammar,linas/link-grammar,linas/link-grammar,linas/link-grammar,ampli/link-grammar,opencog/link-grammar,ampli/link-grammar,ampli/link-grammar,linas/link-grammar,opencog/link-grammar,opencog/link-grammar,opencog/link-grammar,ampli/link-grammar,ampli/link-grammar,ampli/link-grammar,opencog/link-grammar,linas/link-grammar,opencog/link-grammar,opencog/link-grammar,ampli/link-grammar,ampli/link-grammar,ampli/link-grammar,linas/link-grammar,linas/link-grammar,linas/link-grammar