add missing asserts to UserAllocatedEvent class

Improves code quality by adding MBED_ASSERT to UserAllocatedEvent::delay
and UserAllocatedEvent::period function. This prevent changing already posted event.

events/UserAllocatedEvent.h

@@ -215,6 +215,7 @@ public:
      */
     void delay(int delay)
     {
+        MBED_ASSERT(!_post_ref);
         equeue_event_delay(&_e + 1, delay);
     }
 
@@ -224,6 +225,7 @@ public:
      */
     void period(int period)
     {
+        MBED_ASSERT(!_post_ref);
         equeue_event_period(&_e + 1, period);
     }