sat-solver: Minor changes in creating disjuncts
sat-solver: Minor changes in creating disjuncts

- Check lkg->chosen_disjuncts[] instead of _sent->word[var->right_word].d.
- Add a comment on the purpose of _sent->word[var->right_word].d.
- Add a debug message in normal (1) verbosity for now, to notify in case
  of a missing expression. This happens sometimes and leads to a later
  assert "build_clause called with null parameter". The reason is still
  unknown.

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