Parsing JSON serialized payload without protected field can lead to segfault

jwx is a Go module implementing various JWx (JWA/JWE/JWK/JWS/JWT, otherwise known as JOSE) technologies. Calling `jws.Parse` with a JSON serialized payload where the `signature` field is present while `protected` is absent can lead to a nil pointer dereference. The vulnerability can be used to crash/DOS a system doing JWS verification. This vulnerability has been patched in versions 2.0.19 and 1.2.28.