Fix crash when using DHT.

t.M needs to be populated before we can refer to it.

2014/05/04 21:20:03 Starting.
2014/05/04 21:20:03 Listening for peers on port: 7777
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x18 pc=0x3fa6f]

goroutine 16 [running]:
runtime.panic(0x335860, 0x551524)
	/usr/local/go/src/pkg/runtime/panic.c:279 +0xf5
github.com/jackpal/Taipei-Torrent/torrent.NewTorrentSession(0x7fff5fbffb5f, 0x14f, 0x1e61, 0x0, 0x0, 0x0)
	/Users/yves/go/src/github.com/jackpal/Taipei-Torrent/torrent/torrent.go:169 +0x14f
main.main()
	/Users/yves/go/src/github.com/jackpal/Taipei-Torrent/main.go:62 +0x523