detect if peers try to connect over made-up info hashes and ban them

git-svn-id: svn://svn.code.sf.net/p/libtorrent/code/branches/RC_1_0@10458 a83610d8-ad2a-0410-a6ab-fc0612d85776
fix bug in handling DHT error messages

git-svn-id: svn://svn.code.sf.net/p/libtorrent/code/branches/RC_1_0@10459 a83610d8-ad2a-0410-a6ab-fc0612d85776