check deltas for correctness before applying
check deltas for correctness before applying

We can not blindly trust the RCS file to contain correct deltas.  We have
to verify that the chunks (especially deletes) make sense and operate on
existing lines.

Reported-by: Brad King <91bf949d73a8f744e55949ef56efa0d05cf88599@kitware.com>

corecode/rcsparse,corecode/rcsparse,corecode/rcsparse