ovsdb-tool: fix memory leak while converting cluster into standalone database

ovsdb-tool: fix memory leak while converting cluster into standalone database

memory leak is reported by valgrind while executing functional test
"ovsdb-tool convert-to-standalone"

==13842== 2,850 (280 direct, 2,570 indirect) bytes in 7 blocks are definitely lost in loss record 20 of 20
==13842==    at 0x4C2DB8F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==13842==    by 0x45EE2E: xmalloc (util.c:138)
==13842==    by 0x43E386: json_create (json.c:1451)
==13842==    by 0x43BDD2: json_object_create (json.c:254)
==13842==    by 0x43DEE3: json_parser_push_object (json.c:1273)
==13842==    by 0x43E167: json_parser_input (json.c:1371)
==13842==    by 0x43D6EA: json_lex_input (json.c:991)
==13842==    by 0x43DAC1: json_parser_feed (json.c:1149)
==13842==    by 0x40D108: parse_body (log.c:411)
==13842==    by 0x40D386: ovsdb_log_read (log.c:476)
==13842==    by 0x406A0B: do_convert_to_standalone (ovsdb-tool.c:1571)
==13842==    by 0x406A0B: do_cluster_standalone (ovsdb-tool.c:1606)
==13842==    by 0x438670: ovs_cmdl_run_command__ (command-line.c:223)
==13842==    by 0x438720: ovs_cmdl_run_command (command-line.c:254)
==13842==    by 0x405A4C: main (ovsdb-tool.c:79)

The problem was in do_convert_to_standalone() function which while reading log file
allocate json object which was not deallocated at the end.

Signed-off-by: Damijan Skvarc <damjan.skvarc@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>