megolm: Fix use of uninitialized value in group 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.

src/message.cpp

@@ -373,7 +373,9 @@ void _olm_decode_group_message(
     std::uint8_t const * unknown = nullptr;
 
     bool has_message_index = false;
+    results->version = 0;
     results->message_index = 0;
+    results->has_message_index = (int)has_message_index;
     results->ciphertext = nullptr;
     results->ciphertext_length = 0;