Fix #3907

Protect spacer from existential quantifiers in the tail.

Some transformations seem to introduce existentially quantified terms.

src/muz/base/dl_context.cpp

@@ -582,6 +582,7 @@ namespace datalog {
             m_rule_properties.check_existential_tail();
             m_rule_properties.check_for_negated_predicates();
             m_rule_properties.check_uninterpreted_free();
+            m_rule_properties.check_quantifier_free(exists_k);
             break;
         case BMC_ENGINE:
             m_rule_properties.collect(r);