Crash in case which call serViewControllers at the second time.

[Yoshihiro Kato]

Crash in case which called setViewController at the second time in order to change menuBarController's viewControllers.

NSArray* viewControllersFirst = @[vc1a, vc2a, vc3a];
[menuBarController setViewControllers:viewControllers];
...
// Already shows viewControllers in menuBarController
...
// In certain timing,  reset viewControllsers
NSArray* viewControllersSecond = @[vc1b, vc2b, vc3b];
[menuBarController setViewControllers:viewControllers]
// Crash!!

[Yoshihiro Kato]

Pull request #4

[Yoshihiro Kato]

Merged pull request.