Fixed assertion problem in priority queue.

The priority queue relies on setting an invalid time value for
non-existing nodes. However the time comparison checks for valid
embb_time_t types. Therefore prior to doing the time comparison, the
time values have to be checked for validity.

mtapi_c/src/embb_mtapi_priority_queue_t.c

@@ -145,11 +145,16 @@ embb_mtapi_task_t * embb_mtapi_priority_queue_pop(embb_mtapi_priority_queue_t* t
           right_deadline.seconds     = ULLONG_MAX;
           right_deadline.seconds     = ULLONG_MAX;
         }
         }
 
 
-        /* min Heap, swap with the smaller of both children. */
-        if(embb_time_compare(&left_deadline, &right_deadline) <= 0) {
+        /* min Heap, swap with the smaller of both children.
+         *
+         * the separate checks are necessary since the comparison operator
+         * checks whether the time is within certain bounds. */
+        if(right_deadline.seconds == ULLONG_MAX ||
+           (left_deadline.seconds != ULLONG_MAX &&
+            embb_time_compare(&left_deadline, &right_deadline) <= 0)) {
           swap = ii + 1;
           swap = ii + 1;
           swap_deadline = left_deadline;
           swap_deadline = left_deadline;
-        } else if(embb_time_compare(&left_deadline, &right_deadline) > 0) {
+        } else {
           swap = ii + 2;
           swap = ii + 2;
           swap_deadline = right_deadline;
           swap_deadline = right_deadline;
         }
         }