MDEV-19914 Server startup fails while dropping garbage encrypted tablespace if innodb_encryption_threads > 0

- Avoiding accessing encryption thread mutex before initiating
the encryption threads

mysql-test/suite/encryption/t/innodb-encryption-alter.test

@@ -123,6 +123,7 @@ SET DEBUG_SYNC = 'now WAIT_FOR done';
 SET GLOBAL innodb_flush_log_at_trx_commit=1;
 COMMIT;
 
+let $restart_parameters = --innodb_encryption_threads=2;
 --let $shutdown_timeout= 0
 --source include/restart_mysqld.inc
 disconnect con1;