clause_builder should not fail on always-true literals

Otherwise, e.g. when adding axioms, the caller would have to check each literal before adding it.

src/math/polysat/mul_ovfl_constraint.cpp

@@ -138,6 +138,7 @@ namespace polysat {
         cb.push_new(~premise);
         cb.push_new(conseq);
         clause_ref just = cb.build();
+        SASSERT(just);
         s.add_clause(*just);
         s.propagate();
         SASSERT(s.m_bvars.is_true(conseq.blit()));