prevent stale user-propagators from being used on the same tactic aft…

…er it was applied.

src/smt/tactic/smt_tactic_core.cpp

@@ -140,6 +140,7 @@ class smt_tactic : public tactic {
         ~scoped_init_ctx() {
             smt::kernel * d = m_owner.m_ctx;
             m_owner.m_ctx = nullptr;
+            m_owner.m_user_ctx = nullptr;
 
             if (d)
                 dealloc(d);