Fix memory leak when users are dumped

When users are dumped to file, the JSON object wasn't freed.

server/core/adminusers.cc

@@ -82,6 +82,8 @@ static bool admin_dump_users(USERS* users, const char* fname)
         rval = false;
     }
 
+    json_decref(json);
+
     return rval;
 }