fix assert by only constructing piece_block with verified values

git-svn-id: http://libtorrent.googlecode.com/svn@9801 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda
merged changes from trunk

git-svn-id: http://libtorrent.googlecode.com/svn@9802 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda