fix case when zmq_poller access a dead socket

src/socket_poller.cpp

@@ -47,7 +47,7 @@ zmq::socket_poller_t::~socket_poller_t ()
     tag = 0xdeadbeef;
 
     for (items_t::iterator it = items.begin(); it != items.end(); ++it) {
-        if (it->socket) {
+        if (it->socket && it->socket->check_tag()) {
             int thread_safe;
             size_t thread_safe_size = sizeof(int);
 
@@ -648,5 +648,3 @@ int zmq::socket_poller_t::wait (zmq::socket_poller_t::event_t *event_, long time
     return -1;
 #endif
 }
-
-