Fix memory leak on object and associative array processing

Names allocated with `amf0_string_read` inside `amf0_associative_array_read` and `amf0_object_read` are never freed.

The names in question are only used as `(char *)amf0_string_get_uint8_ts(name)`, are copied inside `amf0_object_add` with `amf0_str`, and are safe to free after `amf0_object_add` is done with them.

The same goes for `amf0_associative_array_read`.