nodestack: re-insert falsely-declared "dead code"

nodestack: re-insert falsely-declared "dead code"

"remove dead code...  loop entrance condition is i<backtrack.size().  Therefore,
this branch can never be taken."

False.

While the entrance condition prevents 'i == backtrack.size()', the loop may
exit with 'i == backtrack.size()', since i++ is executed after the last
iteration. Since we do not expect or want this condition to occur, though, I
transform this to a documented ASSERT().