ovsdb-server: fix memory leak while deleting zone

ovsdb-server: fix memory leak while deleting zone

memory leak was detected by valgrind during execution
of "database commands -- positive checks" test.

leaked memory was allocated in ovsdb_execute_mutate() function
while parsing mutations from the apparent json entity:

==19563==    at 0x4C2DB8F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==19563==    by 0x4652D0: xmalloc (util.c:138)
==19563==    by 0x46539E: xmemdup0 (util.c:168)
==19563==    by 0x4653F7: xstrdup (util.c:177)
==19563==    by 0x450379: ovsdb_base_type_clone (ovsdb-types.c:208)
==19563==    by 0x450F8D: ovsdb_type_clone (ovsdb-types.c:550)
==19563==    by 0x428C3F: ovsdb_mutation_from_json (mutation.c:108)
==19563==    by 0x428F6B: ovsdb_mutation_set_from_json (mutation.c:187)
==19563==    by 0x42578D: ovsdb_execute_mutate (execution.c:573)
==19563==    by 0x4246B0: ovsdb_execute_compose (execution.c:171)
==19563==    by 0x41CDE5: ovsdb_trigger_try (trigger.c:204)
==19563==    by 0x41C8DF: ovsdb_trigger_init (trigger.c:61)
==19563==    by 0x40E93C: ovsdb_jsonrpc_trigger_create (jsonrpc-server.c:1135)
==19563==    by 0x40E20C: ovsdb_jsonrpc_session_got_request (jsonrpc-server.c:1002)
==19563==    by 0x40D1C2: ovsdb_jsonrpc_session_run (jsonrpc-server.c:561)
==19563==    by 0x40D31E: ovsdb_jsonrpc_session_run_all (jsonrpc-server.c:591)
==19563==    by 0x40CD6E: ovsdb_jsonrpc_server_run (jsonrpc-server.c:406)
==19563==    by 0x40627E: main_loop (ovsdb-server.c:209)
==19563==    by 0x406E66: main (ovsdb-server.c:460)

This memory is usually freed at the end of ovsdb_execute_mutate()
however in the aforementioned test case this does not happen. Namely
in case of delete mutator and in case of error while calling ovsdb_datum_from_json()
apparent mutation was marked as invalid, what prevents freeing problematic memory.

Memory leak can be reproduced quickly with the following command sequence:
ovs-vsctl --no-wait -vreconnect:emer add-zone-tp netdev zone=1 icmp_first=1 icmp_reply=2
ovs-vsctl --no-wait -vreconnect:emer del-zone-tp netdev zone=1

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