raft: Free leaked json data

raft: Free leaked json data

Valgrind reported:

1924: compacting online - cluster

==29312== 2,886 (240 direct, 2,646 indirect) bytes in 6 blocks are definitely lost in loss record 406 of 413
==29312==    at 0x4C2DB8F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==29312==    by 0x44A5F4: xmalloc (util.c:138)
==29312==    by 0x4308EA: json_create (json.c:1451)
==29312==    by 0x4308EA: json_object_create (json.c:254)
==29312==    by 0x430ED0: json_parser_push_object (json.c:1273)
==29312==    by 0x430ED0: json_parser_input (json.c:1371)
==29312==    by 0x431CF1: json_lex_input (json.c:991)
==29312==    by 0x43233B: json_parser_feed (json.c:1149)
==29312==    by 0x41D87F: parse_body.isra.0 (log.c:411)
==29312==    by 0x41E141: ovsdb_log_read (log.c:476)
==29312==    by 0x42646D: raft_read_log (raft.c:866)
==29312==    by 0x42646D: raft_open (raft.c:951)
==29312==    by 0x4151AF: ovsdb_storage_open__ (storage.c:81)
==29312==    by 0x408FFC: open_db (ovsdb-server.c:642)
==29312==    by 0x40657F: main (ovsdb-server.c:358)

This patch fixes it.

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