fix model return after shutdown, reported in #4532

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>

src/smt/smt_context.cpp

@@ -4486,7 +4486,11 @@ namespace smt {
     }
 
     void context::get_model(model_ref & mdl) {
-        if (inconsistent() || !m.inc())
+        if (inconsistent()) 
+            mdl = nullptr;
+        else if (m_model.get()) 
+            mdl = m_model.get();
+        else if (!m.inc())
             mdl = nullptr;
         else {
             mk_proto_model();

src/smt/smt_parallel.cpp

@@ -163,6 +163,7 @@ namespace smt {
                     return;
                 } 
                 
+
                 bool first = false;
                 {
                     std::lock_guard<std::mutex> lock(mux);

src/solver/parallel_tactic.cpp

@@ -433,6 +433,9 @@ private:
             }
             m_models.push_back(mdl.get());
         }
+        else if (m_models.empty()) {
+            m_has_undef = true;
+        }
         if (!m_allsat) {
             m_queue.shutdown();
         }