The order of acquiring LOCK_mysql_create_db
and wait_if_global_read_lock() was wrong. It could happen
that a thread held LOCK_mysql_create_db while waiting for
the global read lock to be released. The thread with the
global read lock could try to administrate a database too.
It would first try to lock LOCK_mysql_create_db and hang...

The check if the current thread has the global read lock
is done in wait_if_global_read_lock(), which could not be
reached because of the hang in LOCK_mysql_create_db.

Now I exchanged the order of acquiring LOCK_mysql_create_db
and wait_if_global_read_lock(). This makes 
wait_if_global_read_lock() fail with an error message for
the thread with the global read lock. No deadlock happens.


mysql-test/r/lock_multi.result:
  Bug#19815 - CREATE/RENAME/DROP DATABASE can deadlock on a global read lock
  The test result
mysql-test/t/lock_multi.test:
  Bug#19815 - CREATE/RENAME/DROP DATABASE can deadlock on a global read lock
  The test case
sql/sql_db.cc:
  Bug#19815 - CREATE/RENAME/DROP DATABASE can deadlock on a global read lock
  Exchanged the order of acquiring LOCK_mysql_create_db
  and wait_if_global_read_lock().