Add guard on Python single goal action server example (#380)

Signed-off-by: Ruddick Lawrence <679360+mrjogo@users.noreply.github.com>

rclpy/actions/minimal_action_server/examples_rclpy_minimal_action_server/server_single_goal.py

@@ -100,7 +100,12 @@ def execute_callback(self, goal_handle):
             # Sleep for demonstration purposes
             time.sleep(1)
 
-        goal_handle.succeed()
+        with self._goal_lock:
+            if not goal_handle.is_active:
+                self.get_logger().info('Goal aborted')
+                return Fibonacci.Result()
+
+            goal_handle.succeed()
 
         # Populate result message
         result = Fibonacci.Result()