Go JOSE Panics in JWE decryption

QFinch assessment

Decrypting a JSON Web Encryption (JWE) object will panic if the `alg` field indicates a key wrapping algorithm ([one ending in `KW`](https://pkg.go.dev/github.com/go-jose/go-jose/v4#pkg-constants), with the exception of `A128GCMKW`, `A192GCMKW`, and `A256GCMKW`) and the `encrypted_key` field is empty. The panic happens when `cipher.KeyUnwrap()` in `key_wrap.go` attempts to allocate a slice with a zero or negative length based on the length of the `encrypted_key`.

Potential impact

Decrypting a JSON Web Encryption (JWE) object will panic if the alg field indicates a key wrapping algorithm (one ending in KW, with the exception of A128GCMKW, A192GCMKW, and A256GCMKW) and the encryptedkey field is empty. The panic happens when cipher.KeyUnwrap() in keywrap.go attempts to allocate a slice with a zero or negative length based on the length of the encryptedkey. This code path is reachable from ParseEncrypted() / ParseEncryptedJSON() / ParseEncryptedCompact() followed by Decrypt() on the resulting object. Note that the parse functions take a list of accepted key algorithms. If the accepted key algorithms do not include any key wrapping algorithms, parsing will fail and the application will be unaffected. This panic is also reachable by calling cipher.KeyUnwrap() directly with any ciphertext parameter less than 16 bytes long, but calling this function directly is less common. Panics can lead to denial of service.