Fix crash in filesFromStatus: when piece bitfield is empty

[mrowlinson]

Summary

filesFromStatus: calls stat.pieces.get_bit(index) for every piece of every file. When a torrent has just been added and the piece check has not yet populated stat.pieces, the bitfield's backing storage is empty and get_bit dereferences null internal storage, crashing with EXC_BAD_ACCESS / KERN_INVALID_ADDRESS at 0x4.

This patch bounds-checks each piece index against stat.pieces.size() and falls back to @NO for indices the bitfield does not yet cover. Once the piece check populates the bitfield the snapshot is refreshed with the real bits.

Reproduction

LibTorrent-Swift 2.0.11 (commit 70ca1a7) inside a host iTorrent build, on iOS 26.4.2, iPhone 17 Pro Max:

1. Build & run the host app on a device.
2. Add any new torrent (file or magnet).
3. TorrentHandle.prepareToAdd runs updateSnapshot synchronously, which calls filesFromStatus: on a status snapshot whose pieces bitfield is still empty → crash.

Top of the faulting stack from the .ips report:

libtorrent::bitfield::get_bit(int) const
libtorrent::typed_bitfield<piece_index_tag>::get_bit(...)
-[TorrentHandle filesFromStatus:]                                     ← TorrentHandle.mm:398
-[TorrentHandle updateSnapshot]                                       ← TorrentHandle.mm:506
closure #1 in closure #1 in TorrentHandle.prepareToAdd(into:)

exception:
  type: EXC_BAD_ACCESS
  signal: SIGSEGV
  subtype: KERN_INVALID_ADDRESS at 0x0000000000000004

Test plan

• Build passes (xcodebuild -workspace ... -scheme iTorrent -destination 'generic/platform=iOS' build)
• App launches on device after the patch
• Adding a new torrent no longer crashes; piece-status array is populated correctly once the piece check completes