json: improve json parsing

spa_json_get_string() will never write more than maxlen.
Use spa_json_parse_stringn() to perform destination length checking.

modules/module-default-nodes/common.h

@@ -62,7 +62,7 @@ json_object_find (const char *obj, const char *key, char *value, size_t len)
   if (spa_json_enter_object(&it[0], &it[1]) <= 0)
     return -EINVAL;
 
-  while (spa_json_get_string(&it[1], k, sizeof(k)-1) > 0) {
+  while (spa_json_get_string(&it[1], k, sizeof(k)) > 0) {
     if (strcmp(k, key) == 0) {
       if (spa_json_get_string(&it[1], value, len) <= 0)
         continue;