fix assert by only constructing piece_block with verified values

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9801 a83610d8-ad2a-0410-a6ab-fc0612d85776
improve debug output from test_tracker

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9803 a83610d8-ad2a-0410-a6ab-fc0612d85776