ofproto: Fix memory leak reported by valgrind.

ofproto: Fix memory leak reported by valgrind.

Test case 757: ofproto - table description (OpenFlow 1.4)
Call stacks:
    parse_ofp_table_vacancy (ofp-parse.c:896)
    parse_ofp_table_mod (ofp-parse.c:978)
    ofctl_mod_table (ovs-ofctl.c:2011)
    ovs_cmdl_run_command (command-line.c:121)
    main (ovs-ofctl.c:135)
Reason: return without freeing memory

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>