After starting MariaDB 10.2 with an invalid value of
--innodb-flush-method= (the empty string), shutdown would
attempt to dereference some NULL pointers. This was probably broken
in commit 81b7fe9d which implemented
shutdown after aborted startup.

logs_empty_and_mark_files_at_shutdown(): Allow shutdown even if
lock_sys, log_sys, or fil_system is NULL.

os_aio_free(): Tolerate os_aio_segment_wait_events==NULL.

innobase_start_or_create_for_mysql(): Do not invoke
srv_init_abort() before initializing all mutexes for the temporary files.

innodb_shutdown(): Tolerate buf_pool_ptr==NULL.