Fix use of uninitialized value in message decoding.

_olm_decode_group_message should initialize all fields of the results struct before returning. This is because its caller _decrypt_max_plaintext_length relies on it having initialized these fields.

Luckily, this only allows one to subvert the version check in _decrypt_max_plaintext_length, but not the following check that the ciphertext field is non-null because that field is initialized.