row_log_table_apply_update(): Free the pcur.old_rec_buf before returning.
It may be allocated by btr_pcur_store_position() inside
btr_blob_log_check_t::check() and btr_store_big_rec_extern_fields().

This memory leak was introduced in
commit 2e814d47 (MariaDB Server 10.2.2)
via mysql/mysql-server@ce0a1e85e24e48b8171f767b44330da635a6ea0a
(MySQL 5.7.5).