Problem: No coverage for ctx termination errors

Solution:
- Add error state coverage for zmq_ctx_term(), zmq_term() and
  zmq_ctx_shutdown(); zmq_ctx_destroy() is already covered since it only
  calls zmq_ctx_term()
- Add coverage for zmq_term()

tests/test_ctx_destroy.cpp

@@ -50,7 +50,13 @@ void test_ctx_destroy()
     // Close the socket
     rc = zmq_close (socket);
     assert (rc == 0);
-    
+
+    // Test error - API has multiple ways to kill Contexts
+    rc = zmq_ctx_term (NULL);
+    assert (rc == -1 && errno == EFAULT);
+    rc = zmq_term (NULL);
+    assert (rc == -1 && errno == EFAULT);
+
     // Destroy the context
     rc = zmq_ctx_destroy (ctx);
     assert (rc == 0);
@@ -73,6 +79,10 @@ void test_ctx_shutdown()
     // Wait for thread to start up and block
     msleep (SETTLE_TIME);
 
+    // Test error - Shutdown context
+    rc = zmq_ctx_shutdown (NULL);
+    assert (rc == -1 && errno == EFAULT);
+
     // Shutdown context, if we used destroy here we would deadlock.
     rc = zmq_ctx_shutdown (ctx);
     assert (rc == 0);