partial `_check-step-marker_` adds junk to Bool Sort

[Nick Walker]

matching-logic-prover/checker/maude/checker.maude

Line 306 in 9db824d

 ceq Pi1 check-step-marker (((N) Phi Rl ;) Pi2) 

In the case where the left hand argument proof is not valid, the term is not reduced.
The definition for _check-step-marker_ needs a
... false [owise] . equation.
or it could be declared partial
op _check-step-marker_ : Proof Proof ~> Bool .

Reproduce:
red check((0) (\bot) (WF) ; (1) (\bot) (WF); ) .