rxrpc: Fix error handling in rxgk_extract_token()

In the Linux kernel, the following vulnerability has been resolved:

rxrpc: Fix error handling in rxgk_extract_token()

Fix a missing bit of error handling in rxgk_extract_token(): in the event

that rxgk_decrypt_skb() returns -ENOMEM, it should just return that rather

than continuing on (for anything else, it generates an abort).