crash on restarting with an incomplete download

[slingamn]

Getting this deterministically on aa363d8. Reproduction steps:

1. Start a torrent
2. Quit after it downloads some data, but before it completes
3. Restart rtorrent
4. rtorrent starts hashing the incomplete data, then crashes:

rtorrent: caught torrent::internal_error: HashTorrent::queue() invalid handle but no error.
/redacted/lib/libtorrent.so.39(_ZN7torrent14internal_error10initializeERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE+0x90) [0x777c816e90e0]
/redacted/bin/rtorrent(_ZN7torrent14internal_errorC1EPKc+0x88) [0x5ecd40781d18]
/redacted/lib/libtorrent.so.39(+0x451ed) [0x777c8168e1ed]
/redacted/lib/libtorrent.so.39(+0xd637f) [0x777c8171f37f]
/redacted/lib/libtorrent.so.39(+0xb000f) [0x777c816f900f]
/redacted/lib/libtorrent.so.39(_ZN7torrent15signal_bitfield4workEv+0x7f) [0x777c816e1c0f]
/redacted/lib/libtorrent.so.39(_ZN7torrent5utils6Thread14process_eventsEv+0x4a) [0x777c816e399a]
/redacted/lib/libtorrent.so.39(_ZN7torrent5utils6Thread10event_loopEv+0x128) [0x777c816e3c38]
/redacted/bin/rtorrent(+0x53826) [0x5ecd40771826]
/lib/x86_64-linux-gnu/libc.so.6(+0x2a1ca) [0x777c80e2a1ca]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x8b) [0x777c80e2a28b]
/redacted/bin/rtorrent(+0x54685) [0x5ecd40772685]