branches/5.1: Merge r4031 from branches/5.0:
branches/5.1: Merge r4031 from branches/5.0:

btr_search_drop_page_hash_when_freed(): Check if buf_page_get_gen()
returns NULL.  The page may have been evicted from the buffer pool
between buf_page_peek_if_search_hashed() and buf_page_get_gen(),
because the buffer pool mutex will be released between these two calls.
(Bug #42279)

rb://82 approved by Heikki Tuuri


ollie314/server,flynn1973/mariadb-aix,ollie314/server,davidl-zend/zenddbi,slanterns/server,davidl-zend/zenddbi,natsys/mariadb_10.2,davidl-zend/zenddbi,ollie314/server,natsys/mariadb_10.2,natsys/mariadb_10.2,davidl-zend/zenddbi,flynn1973/mariadb-aix,davidl-zend/zenddbi,flynn1973/mariadb-aix,flynn1973/mariadb-aix,davidl-zend/zenddbi,flynn1973/mariadb-aix,flynn1973/mariadb-aix,natsys/mariadb_10.2,davidl-zend/zenddbi,natsys/mariadb_10.2,ollie314/server,ollie314/server,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,flynn1973/mariadb-aix,ollie314/server,davidl-zend/zenddbi,flynn1973/mariadb-aix,davidl-zend/zenddbi,ollie314/server,ollie314/server,ollie314/server,natsys/mariadb_10.2,flynn1973/mariadb-aix,flynn1973/mariadb-aix,davidl-zend/zenddbi,natsys/mariadb_10.2,ollie314/server,davidl-zend/zenddbi,ollie314/server,natsys/mariadb_10.2,flynn1973/mariadb-aix