ovsdb-client: Fix memory leak reported by valgrind.

ovsdb-client: Fix memory leak reported by valgrind.

Free memory allocated from table_add_column and table_add_row.
Test case: vsctl-bashcomp - basic verification/argument completion(7,8)
Call stack is below:
    xrealloc (util.c:123)
    table_add_column (table.c:146) or table_add_row (table.c:172)
    do_list_tables (ovsdb-client.c:449)
    main (ovsdb-client.c:151)

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>
[blp@ovn.org removed an unneeded "if"]
Signed-off-by: Ben Pfaff <blp@ovn.org>