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
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.