Merge pull request #11446 from maciejbocianski/UserAllocatedEvent_update

add missing asserts to UserAllocatedEvent class

events/UserAllocatedEvent.h

@@ -214,6 +214,7 @@ public:
      */
     void delay(int delay)
     {
+        MBED_ASSERT(!_post_ref);
         equeue_event_delay(&_e + 1, delay);
     }
 
@@ -223,6 +224,7 @@ public:
      */
     void period(int period)
     {
+        MBED_ASSERT(!_post_ref);
         equeue_event_period(&_e + 1, period);
     }