rtrlib/rtr: Ensure shadow tables are only freed when initialized (#204)
From the Bug report:
> If rtr_receive_pdu returns an error code, the execution flow is
> redirected to the cleanup label. If ->is_resetting is now true (e.g.,
> because it was set by a CACHE_RESPONSE PDU processed by
> rtr_handle_cache_response_pdu), pfx_table_free_without_notify and
> spki_table_free_without_notify are executing multiple operations on this
> uninitialized data.
This patch ensures the cleanup code can check whether the shadow tables
have been initialized and only frees them if they are.