Crash on ASTableView willDisplayCell: due to nil element

[cesteban]

Since we updated to Texture 2.5, we are receiving a number of crashes on [ASTableView tableView:willDisplayCell:forRowAtIndexPath:], due to nil ASCollectionElement's:

0   CoreFoundation                       0x0000000188c8efd8 __exceptionPreprocess + 124
1   libobjc.A.dylib                      0x00000001876f0538 objc_exception_throw + 52
2   CoreFoundation                       0x0000000188c8ef20 +[NSException raise:format:] + 112
3   Foundation                           0x0000000189687674 -[NSCountedSet addObject:] + 88
4   AsyncDisplayKit                      0x0000000100cac02c -[ASTableView tableView:willDisplayCell:forRowAtIndexPath:] (ASTableView.mm:986)
5   UIKit                                0x000000018f1031b4 -[UITableView _createPreparedCellForGlobalRow:withIndexPath:willDisplay:] + 844
6   UIKit                                0x000000018f10332c -[UITableView _createPreparedCellForGlobalRow:willDisplay:] + 76
7   UIKit                                0x000000018f0f0a30 -[UITableView _updateVisibleCellsNow:isRecursive:] + 2148
8   UIKit                                0x000000018f10831c -[UITableView _performWithCachedTraitCollection:] + 116
9   UIKit                                0x000000018eea092c -[UITableView layoutSubviews] + 172
10  AsyncDisplayKit                      0x0000000100caaf84 -[ASTableView layoutSubviews] (ASTableView.mm:778)
11  UIKit                                0x000000018edbb158 -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 1196
12  QuartzCore                           0x000000018bfab274 -[CALayer layoutSublayers] + 144
13  AsyncDisplayKit                      0x0000000100c1a8a0 -[_ASDisplayLayer layoutSublayers] (_ASDisplayLayer.mm:103)
14  QuartzCore                           0x000000018bf9fde8 CA::Layer::layout_if_needed(CA::Transaction*) + 288
15  QuartzCore                           0x000000018bf9fca8 CA::Layer::layout_and_display_if_needed(CA::Transaction*) + 28
16  QuartzCore                           0x000000018bf1b360 CA::Context::commit_transaction(CA::Transaction*) + 248
17  QuartzCore                           0x000000018bf423c0 CA::Transaction::commit() + 500
18  QuartzCore                           0x000000018bf42e8c CA::Transaction::observer_callback(__CFRunLoopObserver*, unsigned long, void*) + 116
19  CoreFoundation                       0x0000000188c3c9a0 __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 28
20  CoreFoundation                       0x0000000188c3a628 __CFRunLoopDoObservers + 368
21  CoreFoundation                       0x0000000188c3aa74 __CFRunLoopRun + 952
22  CoreFoundation                       0x0000000188b6ad94 CFRunLoopRunSpecific + 420
23  GraphicsServices                     0x000000018a5d4074 GSEventRunModal + 96
24  UIKit                                0x000000018ee23130 UIApplicationMain + 204
25  Tuenti                               0x0000000100321564 main (AppDelegate.swift:18)
26  libdyld.dylib                        0x0000000187b7959c start + 0

After a quick search I found #421, where ASCollectionView is protected against nil elements. I'm lacking some context here, but I assume the same guard should be implemented in ASTableView as well. I prepared a PR with this change.

@appleguy @Adlai-Holler @nguyenhuy please let me know if I'm missing something.