This was reported as Issue #381.

buf_page_set_old(): Assert that blocks may only be set old if
buf_pool->LRU_old is initialized and buf_pool->LRU_old_len is nonzero.
Assert that buf_pool->LRU_old points to the block at the old/new boundary.

buf_LRU_old_adjust_len(): Invoke buf_page_set_old() after adjusting
buf_pool->LRU_old and buf_pool->LRU_old_len, in order not to violate
the added assertions.

buf_LRU_old_init(): Replace buf_page_set_old() with a direct
assignment to bpage->old, because these loops that initialize all the
blocks would temporarily violate the assertions about
buf_pool->LRU_old.

buf_LRU_remove_block(): When setting buf_pool->LRU_old = NULL, also
clear all bpage->old flags and set buf_pool->LRU_old_len = 0.

buf_LRU_add_block_to_end_low(), buf_LRU_add_block_low(): Move the
buf_page_set_old() call later in order not to violate the debug
assertions.  If buf_pool->LRU_old is NULL, set old=FALSE.

buf_LRU_free_block(): Replace the UNIV_LRU_DEBUG assertion with a
dummy buf_page_set_old() call that performs more thorough checks.

buf_LRU_validate(): Do not tolerate garbage in buf_pool->LRU_old_len
even if buf_pool->LRU_old is NULL.  Check that bpage->old is monotonic.

buf_relocate(): Make the UNIV_LRU_DEBUG checks stricter.

buf0buf.h: Revise the documentation of buf_page_t::old and
buf_pool_t::LRU_old_len.