determine_word_expressions(): Avoid a crash without a message
determine_word_expressions(): Avoid a crash without a message

Check for NULL xnode *before* trying to use the xnode.

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