test-ovsdb: Fix memory leak reported by valgrind.

test-ovsdb: Fix memory leak reported by valgrind.

Test case 1205: generate and apply diff -- set -- size (ovsdb-data.at:827)
Call stack:
    ovsdb_error_valist (ovsdb-error.c:40)
    ovsdb_error (ovsdb-error.c:55)
    do_diff_data (test-ovsdb.c:427)
    ovs_cmdl_run_command (command-line.c:121)
    main (test-ovsdb.c:72)
Fix by calling ovsdb_error_destroy() before ovs_fatal()

Signed-off-by: William Tu <u9012063@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>