fixes for +(BOOL)isUnavailable

in CCSectionViewController.xm :
  - isUnavailable should return YES when not available in that case
CCSectionViewController should return nil;
- it could even be removed from here to not initialize the object
without reason and check directly before the allocation, would just be
an optmization, I guess is okay

in CCLoader.xm :
  - if the initializer return nil, set it in the dictionary would cause
a crash