ovsdb-client: Fix memory leak reported by Valgrind.

Testcase 1857: ovsdb-monitor.at:538 monitor-cond-change reports the
following definitely memory leak:
    ovsdb_schema_create (ovsdb.c:34)
    ovsdb_schema_from_json (ovsdb.c:196)
    fetch_schema (ovsdb-client.c:385)
    do_monitor_cond (ovsdb-client.c:1112)

Signed-off-by: William Tu <[email protected]>
Signed-off-by: Ben Pfaff <[email protected]>

ovsdb/ovsdb-client.c

@@ -1120,6 +1120,7 @@ do_monitor_cond(struct jsonrpc *rpc, const char *database,
                                                     NULL, &cnd));
     ovsdb_condition_destroy(&cnd);
     do_monitor__(rpc, database, OVSDB_MONITOR_V2, --argc, ++argv, condition);
+    ovsdb_schema_destroy(schema);
 }
 
 struct dump_table_aux {