Problem: No invalid pointer handling for zmq_poller_fd

Solution: Add some and document it.

doc/zmq_poller.txt

@@ -249,6 +249,8 @@ No registered event was signalled before the timeout was reached.
 On _zmq_poller_fd:
 *EINVAL*::
 The poller has no associated file descriptor.
+*EFAULT*::
+The provided 'poller' did not point to a valid poller.
 
 EXAMPLE
 -------

src/zmq.cpp

@@ -1280,7 +1280,13 @@ int zmq_poller_wait_all (void *poller_,
 
 int zmq_poller_fd (void *poller_)
 {
-    return static_cast<zmq::socket_poller_t *> (poller_)->signaler_fd ();
+    if (!poller_
+        || !(static_cast<zmq::socket_poller_t *> (poller_)->check_tag ())) {
+        errno = EFAULT;
+        return -1;
+    } else {
+        return static_cast<zmq::socket_poller_t *> (poller_)->signaler_fd ();
+    }
 }
 
 //  Peer-specific state

tests/test_poller.cpp

@@ -195,6 +195,12 @@ void test_null_poller_pointers_wait_all_indirect ()
       EFAULT, zmq_poller_wait_all (&null_poller, &event, 1, 0));
 }
 
+void test_null_poller_pointer_poller_fd ()
+{
+    void *null_poller = NULL;
+    TEST_ASSERT_FAILURE_ERRNO (EFAULT, zmq_poller_fd (&null_poller));
+}
+
 void test_null_socket_pointers ()
 {
     void *poller = zmq_poller_new ();
@@ -269,6 +275,8 @@ void test_call_poller_fd_no_signaler ()
 
     TEST_ASSERT_FAILURE_ERRNO (EINVAL, zmq_poller_fd (poller));
 
+    TEST_ASSERT_SUCCESS_ERRNO (zmq_poller_destroy (&poller));
+
     test_context_socket_close (socket);
 }
 
@@ -284,6 +292,8 @@ void test_call_poller_fd ()
 
     TEST_ASSERT_SUCCESS_ERRNO (zmq_poller_fd (poller));
 
+    TEST_ASSERT_SUCCESS_ERRNO (zmq_poller_destroy (&poller));
+
     test_context_socket_close (socket);
 }
 
@@ -656,6 +666,7 @@ int main (void)
     RUN_TEST (test_null_poller_pointers_wait_indirect);
     RUN_TEST (test_null_poller_pointers_wait_all_direct);
     RUN_TEST (test_null_poller_pointers_wait_all_indirect);
+    RUN_TEST (test_null_poller_pointer_poller_fd);
 
     RUN_TEST (test_null_socket_pointers);