MDEV-11688 fil_crypt_threads_end() tries to create threads

after aborted InnoDB startup

This bug was repeatable by starting MariaDB 10.2 with an
invalid option, such as --innodb-flush-method=foo.
It is not repeatable in MariaDB 10.1 in the same way, but the
problem exists already there.

storage/innobase/fil/fil0crypt.cc

@@ -2458,8 +2458,9 @@ void
 fil_crypt_threads_end()
 /*===================*/
 {
-	/* stop threads */
+	if (fil_crypt_threads_inited) {
 		fil_crypt_set_thread_cnt(0);
+	}
 }
 
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