Discard fake events for closed DCBs

If a fake event is sent to a DCB that has been closed, it should be
discarded.

server/core/dcb.cc

@@ -3209,6 +3209,26 @@ static uint32_t dcb_poll_handler(MXS_POLL_DATA *data, int thread_id, uint32_t ev
     return dcb_handler(dcb, events);
 }
 
+static bool dcb_is_still_valid(DCB* target, int id)
+{
+    bool rval = false;
+
+    for (DCB *dcb = this_unit.all_dcbs[id];
+         dcb; dcb = dcb->thread.next)
+    {
+        if (target == dcb)
+        {
+            if (dcb->n_close == 0)
+            {
+                rval = true;
+            }
+            break;
+        }
+    }
+
+    return rval;
+}
+
 class FakeEventTask: public mxs::WorkerDisposableTask
 {
     FakeEventTask(const FakeEventTask&);
@@ -3223,10 +3243,17 @@ public:
     }
 
     void execute(Worker& worker)
+    {
+        if (dcb_is_still_valid(m_dcb, worker.get_current_id()))
         {
             m_dcb->fakeq = m_buffer;
             dcb_handler(m_dcb, m_ev);
         }
+        else
+        {
+            gwbuf_free(m_buffer);
+        }
+    }
 
 private:
     DCB*     m_dcb;