A corrupt Subversion-format delta can request reads past the end of
the preimage. Set sliding_view::max_off so such corruption is caught
when it appears rather than blocking in an impossible-to-fulfill
read() when input is coming from a socket or pipe.
Inspired-by: Ramkumar Ramachandra <artagnon@gmail.com> Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>