Fix downloaded property access
Fix downloaded property access

When bitfield is not yet created, accessing downloaded property will throw an error.
joshterrill/webtorrent,feross/webtorrent,arestov/webtorrent,feross/webtorrent,webtorrent/webtorrent,webtorrent/webtorrent,jamesliu96/webtorrent,jamesliu96/webtorrent,rom1504/webtorrent,joshterrill/webtorrent,yciabaud/webtorrent,yciabaud/webtorrent