This is a follow-up to commit 84e3f9ce
that introduced the EXTENDED log record of UNDO_APPEND subtype.

mtr_t::undo_append(): Accurately enforce the mtr_buf_t::MAX_DATA_SIZE
limit. Also, replace mtr_buf_t::push() with simpler code, to append 1 byte
to the log.

log_phys_t::undo_append(): Return whether the page was found to
be in an inconsistent state.

log_phys_t::apply(): If corruption was noticed, stop applying log
unless innodb_force_recovery is set.