Fix issue #4277: FSFS commit should sanity check representation offsets
retrieved from rep-cache.

* subversion/libsvn_fs_fs/fs_fs.c
  (verify_walker): Add forward declaration.  Assert that BATON remains NULL.
  (get_shared_rep): Verify representations retrieved from rep-cache.

For reference, currently the error is:

    % $svnadmin verify r
    subversion/libsvn_repos/dump.c:1401: (apr_err=160004)
    subversion/libsvn_fs/fs-loader.c:500: (apr_err=160004)
    subversion/libsvn_fs_fs/fs_fs.c:10347: (apr_err=160004)
    subversion/libsvn_fs_fs/fs_fs.c:10309: (apr_err=160004)
    subversion/libsvn_fs_fs/fs_fs.c:4486: (apr_err=160004)
    svnadmin: E160004: Corrupt representation '1 1 16 4 (null) f1d2d2f924e986ac86fdf7b36c94bcdf32beec15 (null)'
    subversion/libsvn_fs_fs/fs_fs.c:4427: (apr_err=160004)
    subversion/libsvn_fs_fs/fs_fs.c:2775: (apr_err=160004)
    svnadmin: E160004: Malformed representation header at r/db/revs/0/1:6
    zsh: exit 1     $svnadmin verify r


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1428700 13f79535-47bb-0310-9956-ffa450edef68