[libtorrent] Bug in mutable dht_get_item when salt is non-empty

[Sivachandran Paramasivam]

There is a bug in dht_get_item that fails to retrieve mutable item with
non-empty from DHT.

When get_item reply is received, the item target id was computed based on
an uninitialised/unused salt variable. Because of this the request and
reply target id didn't match.

Now the uninitialised/unused salt variable is removed and proper the salt
variable is referred.
Pull request is here https://github.com/arvidn/libtorrent/pull/908.

Thanks,
Sivachandran Paramasivam