ovsdb-client: Fix memory leak reported by valgrind.

ovsdb-client: Fix memory leak reported by valgrind.

Test case 1508-1514: OVSDB -- ovsdb-server monitors, call stacks:
    ovsdb_schema_create (ovsdb.c:34)
    ovsdb_schema_from_json (ovsdb.c:196)
    fetch_schema (ovsdb-client.c:375)
    do_monitor__ (ovsdb-client.c:920)
    main (ovsdb-client.c:152)
Fix by adding ovsdb_schema_destroy().

Signed-off-by: William Tu <u9012063@gmail.com>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Co-authored-by: Daniele Di Proietto <diproiettod@vmware.com
Signed-off-by: Ben Pfaff <blp@ovn.org>