buf_flush_try_neighbors(): Before invoking buf_page_t::ready_for_flush(),
check that the freshly looked up buf_pool.page_hash entry actually is
a buffer page and not a buf_pool.watch[] sentinel for purge buffering.

This race condition was introduced in MDEV-15053
(commit b1ab211d).
It is rather hard to hit this bug, because
buf_flush_check_neighbors() already checked the condition.
The problem exists if buf_pool.watch_set() was invoked for
a page in the range after the check in buf_flush_check_neighbor()
had been finished.