Break out of loop if cell not in UITableView view hierachy

This fixes an issue when the cell is not contained in a `UITableView` view hierarchy or does not have any superview at all. I observed this behavior on both iOS 7.0 and iOS 7.1 during scrolling. Perhaps the cell reuse mechanism within `UITableView` causes the cells to have no superview when the `UITableView` puts them in the reuse pool.

In this case, the implementation of `-[BMXSwipableCell didMoveToSuperview]` sets `_tableView` to `nil`. Please have a look if this does not cause any unwanted side effects.