Make registration of unit clauses more robust (#3965)

Make registration of unit clauses more robust (#3965)

Fixes #3959. It can happen that we generate a lemma that results in a
unit clause that matches a unit clause that was added as an input.
However, we are asserting that a unit clause can only be registered as
either one of them. This commit fixes the issue by only registering a
unit clause from a lemma if it is not already satisfied. I chose this
fix because the existing code doesn't seem to do anything (in terms of
solving) for the case where we have a unit clause that is already
satisfied because of an input unit clause.