Since commit 0b47c126 (MDEV-13542)
we treat all-zero pages as corrupted ones.

During a stress test, a read-ahead of an all-zero page was triggered
and the page read was completed concurrently with buf_page_create_low().
This caused the assertion to fail, because buf_page_create_low() was
waiting for the page latch.

buf_page_get_low(): Only invoke buf_pool_t::corrupted_evict()
if the block was not already marked as corrupted.

buf_page_create_low(): On page identifier mismatch, retry the
buf_pool.page_hash lookup.

buf_pool_t::corrupted_evict(): Set the state of the block to FREED
so that a concurrent buf_page_get_low() will refuse to load the page.
Wait for the page latch to be vacant before proceeding to remove
the block from buf_pool.page_hash and buf_pool.LRU.

page_id_t::set_corrupted(), page_id_t::is_corrupted(): Accessors
for indicating a corrupted page identifier.

Tested by Matthias Leich