Fixed intermittent crashes in fullscreen zooming

When zooming in and out multiple times quickly on an image in
fullscreen mode, the example application crashes intermittently.
stops at line:
"for (UIPhotoContainerView *subView in reusableViews) {" from
"UIPhotoGalleryView.m"

breakpoint analyzer:
#0 0x38dd9cc0 in objc_exception_throw ()
#1 0x2e4c99f0 in __NSFastEnumerationMutationHandler ()
#2 0x0009d61c in -[UIPhotoGalleryView scrollViewDidScroll:] at
UIPhotoGallery/UIPhotoGalleryView.m:172
#3 0x311026e4 in -UIScrollView(UIScrollViewInternal) _notifyDidScroll