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/conflict.cpp

@@ -160,6 +160,7 @@ namespace polysat {
         }
         c_lemma.push(c.blit());
         clause_ref lemma = c_lemma.build();
+        SASSERT(lemma);
         cm().store(lemma.get(), s);
         if (s.m_bvars.value(c.blit()) == l_undef)
             s.assign_propagate(c.blit(), *lemma);