[FIX] add NULL pointer check to xTaskIncrementTick()

tasks.c

@@ -4766,6 +4766,11 @@ BaseType_t xTaskIncrementTick( void )
         {
             for( ; ; )
             {
+                /* if the system tick interrupt is started independent from the
+                 * scheduler the first tick increment might occur before the
+                 * the first task is created and therefore pxDelayedTaskList is
+                 * still not set */
+                configASSERT( pxDelayedTaskList != NULL );
                 if( listLIST_IS_EMPTY( pxDelayedTaskList ) != pdFALSE )
                 {
                     /* The delayed list is empty.  Set xNextTaskUnblockTime